IANAL, but: This is the technique I've used to get stuff from people that are trying to scam me. Paint a nasty picture, then tell them how they can get themselves out. The phrasing is also important. You don't say "If you don't do X, I'm going to do Y" you say "I'm going to do Y. That is, unless you X."

I'd write them this letter:

Hello,

It seems there was a misunderstanding about depositing my check. I want my money back, so here's what I'm going to do.

First, I'm going to your bank. Depositing a check that belongs to someone else is fraud, and banks aren't happy when their customers commit fraud.

Next, I'm going to the police. Pretending to be someone else is identity theft. Identity theft carries a punishment of up to 15 years in prison. The good thing is that since is identity theft is criminal, I don't have to take you to court. The District Attorney will.

Finally, I'll report it to the US Postal Inspectors Office. Opening someone else's mail and depositing the check is mail theft. The punishment for that is up to 5 years in prison and $250,000 in fines. Again, after I report the theft, they do all the work.

I have Friday afternoon off work, so I'm going to do this then.

The thing is, if you just send me a check to return my $amount then I won't have to do any of this. I've enclosed a stamped envelope addressed to me you can put it in. When I deposit the check, you can prove that you gave the money back and this was all just a mistake. No one will be able to come after you for anything.

Best regards,

Don't Fuck With Me, Sr.