I'm surprised you tolerated it this long. I had a behavioural child stay for 9 months, and each day I was stressed to the max and the rest of the kids suffered from the stress too. I vowed I would never do it again-it's not worth the emotional toll that it takes on everyone. Set a timeline for a probationary period to see improvement, and if it continues, then it's time to let them go. We can't be everything to everyone, and sometimes have to realize that it's not going to work.