We sent him a warning letter (at our lawyer's suggestion) after a bad outburst about a month ago.

Tell him you will call the licensing authority, write scathing reviews, warn people at REIA meetings, write on every GC/developer's FB page warning them about this jackass etc.  

WARNING- Real Estate is NOT the "get-rich-quick" thing that a lot of newbies think it is.

If they've been asking 15K for a while that would be a HUGE warning sign that the property is not worth that.  

So he called himself out.I called the real deed holders to warn them that someone was trying to sell their home and they were thankful we called.

Then if you said no, I would stamp my feet a bit, sigh loudly, mutter a bit, possibly throw a stink eye and warn you that this thing may just fall apart.

Warning: Stay away from MU Law School students.

Often just letting other side know you're represented (e.g. a stern lawyer/demand letter) can end the matter.

It's great to have investors with decades of experience give me advice when I know there are so many other things you could be doing with your time and there is no reason why you should warn me about future pitfalls in this field.

As most experienced investors will warn you here on BP, avoid the gurus.