you could say : STOP STALKING ME!
okay okay, how bout this, give him your number!
but not just any number! the REJECTION HOTLINE!
http://www.rejectionhotline.com/haha, seriously, "sorry (most sincere voice), i made plans to go with my friends. (give him a big smile) and WALK AWAY!!
its better have a friend with you if you sense impending doom. feel me?