Did he do anything before getting on probation? Also if I am not mistaking K2 is legal, but its certainly not safe in any way as far as I know....any ideas as to why he feels the need to resort to that? is it an attempt at self medication or just to get high?

Also it is your house so its your decision if you want to evict his uhh ***, but if he has severe disabilities enough to warrent social security income it is likely he'd have nowhere to go and could end up in even worse situations or even dead. I unfortunately do not really have a solution but maybe others here will have more useful advice.