IANAL and IANYL
FWIW, my SO and I both had a very good laugh at it, and neither one of us think it could be construed as a threat. 
And yes, “I hope you get ass cancer,” would have been “needlessly unprofessional”, (just slightly, though.) 
Well, I’m Canadian - it’s possible that we enjoy a more comfortable reasonable-person-to-hysterical-idiot ratio here, and I am spoiled by that. 
Not only that, but I understand that our court system also reflects that (as in, any court case that they might try to bring against you would simply be thrown out as frivolous). 
I envy my neighbors in the ‘Great White North’.
I’m Canadian and American, both. In my experience, the northern citizenship doesn’t have a terribly favorable “reasonable-person-to-hysterical-idiot ratio”, either. Some of my cousins demonstrate that Canada ain’t the wonderful place you claim. You’ve got plenty of idiots. And half of them are my cousins.
And guess what? I can move to your town tomorrow. Get over yourself, Canada-snob.
But we do still have a favourable un-bunched panty ratio! 
This would have been a good way to end the letter. In a situation like this, I wouldn’t be overly worried about professionalism, only that you’re making your point in a clear and unambiguous (and lawful) manner. A statement like this would have allowed you to get in your kicks without conveying any menace.
Why link no work?
If you’ve already notified them to not bother you (in writing or other provable means) and they kept doing it anyway? According to the Fair Debt Collection Practices Act, you might be able to get a $1000 payment from them.
That is an American act, and does not apply to the OP’s location.
I dug it I’m still giggling over the “at your earliest convenience” part.
And hugs Sorry about your mom, hon. I lost mine in 2011 too.
Larry, as you may know, your remedy lies in the British Columbia Business Practices and Consumer Protection Act S.B.C. 2004, c. 2. Specifically, s. 114, on harassment:
… and s. 117, on communication with persons other than the debtor:
I know you cc’ed Consumer Protection BC in your fax, but you may want to follow-up with that agency to see if they might intervene for you.
People, please, a little common sense!
Larry sent this via fax. Fax! In 2012, the idea of a fax is so quaint. But it’s good news for Larry. Since it was sent via fax, there is absolutely no way that any part of the message, including the death threat, will be legible.
Larry, compose your message with a word processor, print to a .pdf, and e-mail the death threat.
Just kidding. That would be lame.
It’s 2012 - just tweet the damn death threat.
Finally got to see it (had to register with ImageShack because it said “unregistered domain” - wtf?). The letter is a work of beauty and an example to us all. Congratulations.
I’d have split it away from the bit about not contacting you. Obviously anyone with an average level of reading comprehension would understand that you’re not actually saying “Don’t contact me again, or I’ll burn down your office with you inside it”, but the other half of the human race might make the mistake.
Larry, that letter was exquisite! Well done!
When My grand mother died, my uncle had the same problem. he sent them the death certificate and her “new” address, at the cemetary. They never called back. Maybe he just got lucky.
That seems a remote enough possibility that I’m comfortable with it. It’s clear from context that there is absolutely no expectation that they would attempt to contact me further, and so (unlike them) I would have no need to resort to vague threats in the hopes of influencing future behavior. The confidence in taking the liberty of using language like that flows directly from the absolute security that there will be no more communication from them.
It would take a lot of willful ignorance to pretend to parse that as a threat. Seriously, if I sent my next door neighbour a letter reminding him of the bylaw requiring him to maintain his lawn, closing with “I’d be very much obliged if you would be so kind as to mow your lawn at your earliest convenience,” I would not worry that he might interpret it as an offer to mow his lawn - and the substitution of the phrase makes it no less clear.
Even if someone were to fixate entirely on that phrase, its literal use is overwhelmingly overshadowed by its well-established use as an expression of dismissive contempt in the same category as “Go fly a kite!” or “Get bent!” but conveying an intensity which is appropriate for the sort of person who repeatedly threatens bereaved persons in the hope of tricking them into assuming responsibility for the debts of deceased family members who don’t even live in the same city.
I would be more careful in my communication with them if I didn’t have the security of knowing that they are absolutely in the wrong and could be fined for pushing it any further. Apart from the strong disincentive, they have absolutely no incentive to do so, either - they were just shaking the tree to see if anything fell out, and they are assured there’s no fruit there.
I derive a certain glee when truly reprehensible people go far enough outside the rules as to give me the opportunity to dispense with the usual polite masks and just give them both barrels. Last week I raised some eyebrows at work when they heard my half of a telephone conversation:
The other side of the conversation, which gave me the license to tell them what I really thought (without the usual concern for our corporate image,) was “Are you the person responsible for purchasing office supplies for your company? […]I need you to read me the model number on the front of your copier.”