Were any of the complaints about their children? Whatever you do do not discuss this any further with them and do not get involved in the other tenants issues with them at this point in time. Just issue a 30 day notice. Your other choice is to up their rent and then maybe they will want to move. I think they would have a very hard case to prove it was discrimination, most likely they are just bluffing and hoping you won't call their bluff and let them stay. If they do sue will any of your other tenants be willing to go to court on your behalf?