"What's seventeen inches and resting on my thigh"?

Take out a piece of clear tape, put it where they touched the screen, take it off and put it on a blank piece of paper, then write their name and the finger they used on the paper. If they ask say it's "for my records."

In my opinion comedy only works in situations like these if you're also a bit silly/comic in other situations. Otherwise it's obvious that you're just using it to try to diffuse the confrontation and it doesn't work.

I don't think your favorite line will work if it's a female co-worker though !

That is definitly one to remember. And it is actually good advice, making a joke out of it will make the other person feel less bad.

Ok, if someone used your favorite line on me it would provoke an entire hand rubbing across the screen, but then I'm a bit of a real life troll. :-)

Or if they're wearing glasses touch those with your greasy fingers. They'll get the hint.