How can I tweak my internet network a bit?

Hey All,

I have a DSL modem, hooked up to an Apple Airport Extreme. I have two networks, mine, and a guest network.

Two questions:
1) How can I (if at all) control how much internet usage the guest network consumes?

2) How can I view if anyone else is on my networks? It is invisible and password secured, but I just want to see if anyone possibly got on. Also, if that is the case, how do I block them/ kick them off?

Thanks.