SO my roommate has this friend kate. And I really am not a fan of Kate's what-so-ever. When she's around my roommate makes BAD decsions. She also acts like the queen of the universe when kate's around, because that is what kate thinks she is. I've told my roommate this many times, and she listens but it's like she doesn't hear me. But here is the problem, I don't want Kate at our house. She always has weird and not so good people with her and then she acts like she's owns the place. But how could I nicely let my roommate know that I don't want her here period without coming acrossed and a jerk? Also Kate always seems to make plans with my roommate and then either cancels or doesn't show up then will randomly show up unannounced. How do I deal with this the nicest way?

You will have to let your room mate make her own decisions of who she is friends with and who she is not. If Kate is not a good friend then your roommate may come to see this herself, she may not though, and you will have to accept it. If you do not want Kate at your house you may have to move into a house on your own then you can decide who comes in and who doesn't. If your roommate wants kate there then I don't think you can stop her.