Gaming - what should A do?

Anyone help, please?

As many of you know, I play RPGs, mostly D&D and Call of Cthulhu. I'm a GM for both.

I have never previously encountered someone who is actively cheating. But I believe one of my D&D players has read the scenario I'm running in advance and is taking advantage of that knowledge.

I have no idea what to do, other than to boot him out.