If they are creeping you about, they are not flirting. They are being creepy. You should, and can, enjoy being flirted with. If you don't flirt back, the guy shouldn't expect ANYTHING from you. Anyone who asks you directly to perform a sexual act when you all you've been doing for the past 10 minutes is talking and who KNOWS you are underage isn't flirting, they are being immature and creepy. Flirting is defined as playfully showing your sexual attraction for someone. You should be amused and flattered by flirtation, if it's creepy it is not FLIRTY.