I'm going to blow up on my roommate's friend.
What did your roommate's friend do to make you mad?
It's because he always comes to our house when my roommate's away and I don't like that.
I understand how you feel, but you shouldn't overreact to your roommate's friend. Just politely tell him that your roommate is not currently home and he can come back later.
Oh thanks for the advice. I'll give it a go.
I'm sure he will understand. He has other places he can go to I'm sure.