So, need to know what was pulled back or the cause of the backup before any decisions can be made.But, if they are putting things down that don't go down you should tell them you got this one, the rest will be on them, give them a warning, get a report from the plumber that says exactly what it was and that is is cleaned out now so you know the next time if the same stuff gets in its theirs.  

[But, when you see it happening, you'll also become aware that the warning signs were there all along!] 

The last home inspection I did I got several safety hazard warnings show up due to not having the appropriate GFCI outlets in the right place.

Sadly, no one warned me about the dryness and text-heavy teachings of REX.

I assume maybe this is a good warning sign?

Simply warning them is not enough to protect you from liability.

This is also a good idea although I would warn you about over leveraging so make damn sure you have a good spread in the deal if you 100% finance.

Warning! 

Hey Local BP'ers,Must warn you, this is a lengthy read but an interesting scenario that could happen to you. 

Serve him a NO Trespass notice, warning him NOT to come on property again.