I have not had a visit from the JW’s where they did not leave when I asked them to. (Mine never asked for cash, either.)
You’re not going to confound them with something they haven’t heard rebutted before. You’re not going to un-convert them. Unless you wish to impress yourself, why waste your time?
What exactly does she want you to do to “deal with” them?
You would ridicule a door-to-door salesman who is perfectly content to leave if you simply ask them to?
If I say “no thanks” and they leave it at that and walk away fine.
Ridicule is not my opening gambit. It is the one I fall back on if the person in question continues to bug me after being politely asked to go away from my home.
It seems like the original post was written because the roommate in question wants them to leave, but simply can’t bring herself to ASK them to leave. Escalation from passive compliance to open ridicule has to pass through polite insistence, first.
The last time a pair came to my house, I said, politely, “I’m sorry, this is a devout atheist household,” and they ran. It was true also.
I don’t think the Catholic gimmick will work, since then you’ve said you’ve bought into the bit. When the Baptists came and I told them I wasn’t interested because I was Jewish, they started to try to save me. The atheist bit worked much better. We’re eeeevil you know.
Greet them at the door. Rub your hands together, cackle evilly and say something like : “The Dark Lord will be pleased with tonight’s sacrifice, BWA-HA-HA-HA !” 
Baptists aren’t JW. I’m all for shoving the Bible up the noses of Baptist doorknockers letter by letter, preferably in the original metal type.
I was raised a JW (and left voluntarily at around 14 when I finally had the courage of my convictions to say that I didn’t believe any of it and didn’t think that was likely to change) and did the pioneering thing a couple of times.
I’m sure that quality of response and persistence varies a lot from person to person just like in anything else. But I can say that any rude or inexplicable behavior isn’t likely to be new to them. For us they were just fodder for amusing anecdotes when the various groups gathered for lunch
Frankly, it is an oddly dickish form of passive aggresiveness on the part of your roommate if she isn’t willing to have you simply say “thank you but we’re not interested, please don’t return” but she is ok with you trying to say the worst possible thing mostly likely to “scare” them away.
And if she’s the one who’s been engaging them in conversation then you popping in to say “go away” isn’t necessarily likely to help unless she agrees publicly. They believe they are bringing salvation, why would they let you deny it to her when she appears willing to at least minimally engage.
Really, she needs to just get up the nerve to say to them “Thank you, I’ve appreciated your information but I’m really not interested and please do not come back.”
If, after that, they still do return, then other reactions might be more appropriate.
You can try channeling Fred Phelps and denounce their denomination as pro-sodomite fag-lovers.
Or you could put on your own white shirt and tie and and act as though they came back to hear you preach to them about the Book of Mormon.
Too bad you don’t have an e-meter or you could really have a lot of fun trying to convert them to Scientology.
Or you could do a bit based on the scene in Life of Brian about saying “Jehovah”.
Or you could make a joke about the Jehovah’s Witness Protection Program, or say, “Boy, I bet we could save a a lot of money if we could get you Jehovah’s Witnesses to conduct the census or deliver the mail”.
Or as a last resort, cough and sneeze a lot while making subtle references to the swine flu.
A JW spent a lot of time working on my grandmother, who didn’t mind the company but who (I’m glad to say) wasn’t the least bit interested. So being clearly Jewish wouldn’t have worked for them either, I suspect. I don’t think either is coming back to my house for a spell. so I can’t finish the experiment. Too bad - I’d love to get one into an evolution discussion.
I’ve always had questions about all of the incest in the bible normally it takes a while but they start to get offended and leave, normally right about when I start talking about Jesus having sex with Mary. I find it an entertaining half hour.
A couple of quick points;
The Earth started with a limited population
The flood killed every one but one family
Jesus is one and the same as god the father who exists at all places at all times
They came. I had a bit of fun questioning them on why should I believe their One True Belief when there are so many others out there, but I’m not very good at verbal debate. I don’t think on my feet all that well, which is why I asked for help ahead of time.
I did pretend we were short on time, and stongly insisted that they call my phone, not my housemate’s spouse’s phone before arranging a return visit. And hmm… I might have accidentally given them my old cellphone number rather than my current one… Oh well. 
Thanks for all the responses.
*** Ponder
Yes, this is the classic, SDMB-approved response. LINK, for those unaware of the incident in question (it’s Post #18 from this thread).
Darn, I missed your window of opportunity.
For all religious types that have the new testament somewhere in their teachings, I simply tell them to read Matthew 6, verses 1-8. And then say wow, I really feel sorry for you now, because now you have no heavenly reward. Dude. Sorry.
Don’t forget the Invisible Pink Unicorn.
Answer the door wearing nothing but a leather g-string and a coating of oil, embrace the JW’s hard, and scream joyfully, “I KNEW you’d be back!”
This is the only way to greet anyone trying to sell you something at the door.
Your friend invited them in. It’s her problem, not yours. If she can’t say no to someone selling GOD door-to-door, then she has a problem. Rescuing her is not your problem.
If you have decent knowledge of their beliefs, and neighbors you get along with, you could join them. Leave your house and walk along with them to the next house.
They say to your neighbor “We’d like to speak to you about x.”
And you add “And I’d like to speak to you about why x is wrong.”
Or maybe just “Yes. We’d like to speak to you about x.”
And keep repeating whatever they say, prefaced with “Yes”.
How is going door-to-door and (annoyingly) attempting to convert people the least offensive version of Christianity? To me, the least offensive is the one that can keep their damn mouth shut about their silly beliefs. Their actions are downright rude.