Well fatman...I'm glad you asked.
Noobs are found everywhere aren't they? And for those of you wondering how to identify a noob here's how.
1. A noob will run up to the witch in L4D and say "why are you crying?"
2. A noob will generally be the person who either blows himself up or impales himself on a crate
3. A noob will ask questions like "what does LOL mean?" to which you should respond with "stfu"
4. A noob is a person who will write self help sections in order to sate my...I mean his...madness
5. A noob is a person who will attempt to build a sentry gun on top of a toilet
Now to answer the question of how to politely tell one of these shy and somewhat cuddly creatures to piss off?
There are 2 guaranteed ways of making them stop gaming with you.
1. Tell them to ask their wife/GF/mother if they could supply the condoms tonight as you've lost your wallet...then rock up to their house later that night wearing cheap aftershave, with your dick hanging out of your pants, holding a strap on in one hand and a tube of anal lube in the other
or
2. Spray paint this logo in game on a wall and direct their attention to it.
Note : I suck at paint and you can insert name where appropriate.

Hope that helps,
Chalice,
Lord of Noob