How to turn a guy down nicely without embaressing him?

im being chased after by two guys (both of which are wayy hot but arent for me). I mean, i'd say yes to one of them because it would be great for my reputation but... I just dont know why i dont want to. Anyways, what do i say? I've been asked out 8 times before in my life. 7 times i said "no, thanks" and 1 time i said " im calling the police" but thats not very nice i know realize. I'm 15 btw. Maybe i should stop wearing low cut shirts. Anyone know where i can buy a minimizer bra?