From what you have described, I don't think he will stop, but if you begin with that first step, you've at least left open the possibility of him realizing his mistake and correcting it without threats which are going to cause the relationship between you and the tenant to sour, make your living situation miserable, and encourage him to trash his unit or create other problems.

There were some threats of lawsuits from one guy because he had a competitor try to buy his share out to gain an edge against us, but we were in the right and kept pushing and everyone backed off. 

After reading about the guy's threat regarding RESPA violations, my instinct would be not to do any business with him.

Even if you're 110% honest and up front, there will still be questions, maybe hard feelings, and hopefully never litigation/threats/etc.

Sounds like you need a restraining order or at least the threat of one.

My mother works maintenance at UMass in the dorms and she says those kids make a flipping mess despite threats of being written up or fined.

I was merely trying to correct some misinformation on this threat about UBIT always applying, about funneling income out of the IRA, and about LLCs being required.  

NYC real estate will only continue to go up over time bar any major outside threat attacking it.  

Engaging in acts of violence or threats of violence, including but not limited to the unlawful discharge of firearms, prostitution, criminal street gang activity, intimidation, or any other breach of the Lease that otherwise jeopardizes the health, safety or welfare of the Landlord, his Agents or Tenants.

I will be doing lawyer letters direct to the tenants ordering a cease and desist with the threat of a lawsuit for lost rents to follow if nothing changes.