Ye little drawing board. Pom's tutorial. help?

I’ve been trying (and failing) now for a while with something related… (granted, I am a newbie)

the tutorial is easy and well written, that’s not the problem. the problem is I’m trying to have the drawing board a part of a movie without letting people draw all over the whole thing. Is there any way to limit the x and y coordinates inside which it is allowed to draw?

cheers
/Daniel