I suggest a sign…
http://images0.cafepress.com/product/264141720v4_225x225_Front.jpg
Pass a copy of your OP to each guest as they arrive and be sure to highlite the part where you mention clogging your own toilet each week…
Pretty sure that would do the trick!!!
But seriously, a note on the door saying that this particular toilet is a no-go would be just fine, everyone would understand and be fine using the other two toilets.
A previous post may help.
I think she said that another one would be available. Unless I mis-read that.
Hah, Cicero, I immediately thought of that thread too.
You haven’t seen the size of her BMI’s, though.
I love your search terms. 
And that’s exactly why I warned against illustrations.
Heh. I immediately thought of that thread too. I was thinking “Oh no! Not another one!” I was quite relieved (ha!) to see that it wasn’t the same type of issue.
Well, in honor of Shat on Cookie, perhaps you should put up a sign that says:
No Taking a Dump and/or Whacking Off
You can die of human waste implosion for all I care
Ooops- caught out 
I don’t think you can tactfully explain this. If it’s in a bathroom and flushes and has a seat, anyone and everyone will think they can use it for dumps. It’s what a toilet is for. For the same reason, trying to hint via requests not to throw in sanitary napkins or Kleenex would be pointless, since these are often posted in public restrooms with no intention of discouraging people from using the toilet for both of its primary intended purposes.
Oh, that was lovely, wasn’t it? “How dare those lower life forms use MY royal throne! The utter nerve! I haven’t been able to move off my fainting couch for two days now!” Good times, good times.
I thought those were BMII’s.
That was the last thread she ever posted in, too. Unless she changed her username due to the ridicule.
If someone has to shit they have to shit. Where, besides the toilet, should the deposit be left?
:eek:
Seriously, your toilet - even an old toilet - shouldn’t be THAT easy to clog. Realistically, though, you can’t really STOP guests from shitting if they have to shit. You might wish to post a sign saying something like “Old plumbing - please flush early and often and take care not to clog anything”.
“Anything over 10 pounds lower by ropes”.
At least your guests aren’t wiping themselves with the hand towels.
Are you only worried about people using the one bathroom or do you not want them dumping in any of them? If it’s just the one either put a lock on that door so you don’t have to worry about it or close the lid, set the plunger on top of it and put an “out of order” sign on it and most likely no one will touch it.
Remember this sickening sign?
Hang one that says this.
You don’t have to say anything, just remove the toilet seat. Direct anyone who finds the bathroom unusable in this state to the other bathroom.
In practical terms, your best option would be to have a plumber snake your toilet pipe before the party. Just consider it part of your party planning.