Girls, how the hell are men supposed to resist !?!?!?

ok, so here is the story, my best friend WAS going out with this girl, but about 2 week ago they broke up, and to be honest i like the girl, she is funny,pretty and smart, but i don't want a girlfriend right now and i don't wanna stab my best friend in the back. how ever, about a week after they broke up i was walking to my next lesson , and i took a route that is slightly longer, but less used, so i was the only one on the corridor, and i see my friends ex, she notices me and grins, and walks up to me, presses me lightly against the wall, and whispers in my ear " don't tell (best friends name)" hugs me and walks off. at this point i feel tiny, and my legs feel like jelly. i dont wanna backstab my friend and i dont want a girlfriend at the moment, so how the hell do i cope with another incident like that should it happen!?!?!?