Just don't show interest for Michael.
If he tries flirting with you, ignore him.
And if he asks you out, respectfully say "No thank you." don't mbe all "NO!! D:< I HATE YOU!!! GO AWAY!! RAWR!!"... If you be mean, he may go behind your back and tell Nick. He may say stuff like how you rejected him, which WILL make Nick say no [And it would be a BIG TIME turn off to have a mean girlfriend] when you ask him out.
Also, if your friend's with Michael, just tell him "No thank you, Michael. I'd just like to stay friends.".
Good luck