attention males

[______blackout]

this is going to sound conceited no matter which way i put it, so bear in mind that I AM NOT IN THE SLIGHTEST. anyway, i seriously need help with letting members of the opposite sex know that i am not interested in them. i can't do it! i can't turn guys down. i like to keep feelings intact, what can i say. example: this guy keeps calling and texting me, i just respond nicely because i don't know what else to do, and now i'm sure he thinks i like him although i'm already basically in somewhat of a relationship.

guys, TIPS, ASAP, please
(seriously)
what's the best way for a girl to say she doesn't like you?