What are some good ways to get revenge on a roommate for abruptly kicking someone out?

First, what he is doing is illegal, so report him to Children's Aid. That money is for people who actually need it, not just lazy bums. Second, if your boyfriend had an actual rental agreement/contract with him, he has several legal recourses.....he could go to the landlord. But if the landlord did not approve his living there, your boyfriend has no choices, other than learn from this and cut the friendship off.