They've been threatening to do so for a while. . .I don't have any STRs, so I am simply curious what the latest is.  

When we had apartments every month someone was losing their temper and threatening to sue me or to talk to their lawyer.

All you "owners" who need the ego boost from telling people that you are the owners will change your tune quickly the first time you are threatened or attacked by some wackjob. 

Quote from @Derrick Ruiz: Guys you are crazee if you think you can strong arm tenants or threaten them in any way and get a way with it.

Or, having lawyers call him to threaten him with legal action.  

Don't threaten me with a good time working for peanuts, because I only do what I love.  $$ is really inconsequential in what I do.EDIT:  A lotta you guys remind me of the waiters, waitresses who complain that they aren't paid a minimum wage... but then a law is passed that forces restaurants to pay them a living wage, they FIGHT AGAINST IT!! 

As others have suggested, you could threaten to report the incident to the commission. 

Make sure you are concise and read your letter the next morning before sending to make sure there isn't any tone in there that could be implied as angry or threatening.

Dennis,you have made several mistakes and now is the time to fix this mess before it gets any worse.Go up to the door,smile big,and politely ask what is the problem.When they give you their sob story tell them nicely you can afford to keep them anymore and offer them their deposit back and some extra cash to vacate as soon as possible.Don't get into an argument or makes insults or threaten legal action,just make the offer and leave without any confrontation.Sometimes a little extra cash is all it takes to get rid of them without an expensive,destructive,9 month drag out legal fight.Tell them just to pack up and leave and you will clean up after them( you're gonna do it anyway).

Start threatening to sue the contractor for failure to perform... it probably won't help but it may push him into motion to cut you a check.