I'm already spoken for.
I'm already taken.
When somebody say you "I like you."
But you already have a boy/girl friend.
Then you say "I'm already spoken for." or "I'm already taken."
bonus
Steve Wariner "I'm already taken"
http://kr.youtube.com/watch?v=ObowphKhKn0
related link
http://blog.naver.com/raonbooks?Redirect=Log&logNo=20050027475
