Well, anything but confusion. [Here, have some more stuff to confuse you.] If you don't find him attractive at all, stare judgingly at him or look disgusted. You can give him a polite 'fuck off' smile, maybe. I think you'd do well with a smug look. It says, "I know you want to have sex with me but you're beneath me." That holier-than-thou approach suits you.

[Pause] Unless you do want to get close to him, then by all means sit on his lap.