Have your other tenants filed formal complaints with you? Do they have proof? If you can get them to file complaints with proof you will most likely have a case for eviction. What province are you in?
Another option - get a security camera installed that emails pictures directly to you based on movement. My neighbor just installed one and now he sends me pictures whenever my dog gets out of the fence and into his yard (how wonderful!!). It was cheap, works great and you could potentially set up your own "sting" operation - put another lawnmower where the last one was, or something else, set up the camera and wait...if he takes it there`s your proof - call the cops and file for eviction.
Also, make sure you read up on the tenant/landlord laws in your area so you`re the authority on the matter...not his sister!
Last option, bribe him out - $XXX dollars paid to him once his crap is packed up and out, get him to sign something stating he has decided to end tenancy and he will not hold you liable for any damages regarding finding new tenancy, moving costs, etc...will it hold up court, no clue, but get him to sign something and only pay once he is packed and you have the keys (have his sister sign it as well).
