Exclude certain categories from main page?

Hello all,

First of all, I’ve coded a new theme for my blog over at – http://www.blogmastr.com/. And I managed to do this little nifty thing where the two top boxes (Art showcase and site showcase) Show some data from random posts from those two categories respectively.

That’s all good. But now, what i want to do is exclude those two categories from the main content.

I’m sure there should be some relatively simple way to do this but I can’t figure it out. Any help appreciated, Thanks!