How to reject a guy, nicely?

this boy just asked me out over twitter and schools out and im not into the summer rerlationship thing and even if I wanted to date him I wouldnt cuz we would never see each other and he had to just ruin a great friendship by asking me out now its weird and I like someone else too so how can i put it nicely? would it be rude if I just never replied... and I'm not used to guys asking me out and I feeling he liked me too. I thought my feeling was wrong...