How to stop liking someone

i like him and he likes me but hes not really that good and he has millions of girlfriends I went out with him before

The only thing I can say is just to try and forget about him, if that's what you want. I have been in almost the same situation before and the only way out is to forget about him.
When you meet someone you are attracted to and you don't want to be, you have to just tell yourself that it's probably just hormones taking over. Yes, you may feel somewhat empty and you may miss him and everything in your heart (or otherwise) may tell you to call or go see him, but you should do your best not to do that. It won't be easy, like most things you do that are good for you.
You have to remember why you feel you should not like him in the first place. You said it is because he is no good and he goes out with a lot of other girls right? Remember that every time you see his bedroom eyes and strong, beautiful lips in your daydreams. Remember that girls like us are attracted to bad boys that we cannot tame. It's the thrill that your attracted to, and the fact that you will probably never nail him down to make house with.
Maybe you need to just go out and find a nice guy that reads and appreciates you for the good girl that you are... Nice guys can be really good in bed anyway. They really want to please you, but just don't take advantage of their courtesy too much.