Noob should be a player that has very little experience with the game or is a weak player. I'm actually fine with a player being a in game with 300 games that is a noob. Maybe they actually are a weak player or casual or someone that doesn't get how to play well. If they have more than a 40% win rate, I don't think they are a noob though, but somewhat close if there are idling around 40%.
Now, if we are talking about higher level games, I generally host mid+ games (which means mid level (players that can generally hold there own, but are probably lacking in a few areas) and above (the really good folks that I know will give me a challenging game if I have good teammates). As my game name (mid +) indicates that I expect mid level and above players only to join, if someone with only a few games joins, I will generally ask them to leave and then kick if they don't. To be more specific, most of my hosted games are RANDOM ASSASSIN MID +. And these new players will join, not go random, etc. Which means they didn't even read the title and to me, that means they can't possibly be a mid level player... if that makes sense. And that said, the community is pretty small - I've bumped into pretty much all of the experienced players at one point or another, so I have a pretty good idea of who fits in and who doesn't.
Anyway, looking at it from a step away, if I was a brand new player to the game and someone with 500 games joined my noob only game, I'd kick them. That might be the best course of action for you while you are figuring things out. So, my simple advice to you is host your own noob only games (even if you can beat people that think they are good) and kick folks with many games until you get used to the community. It won't take you very long to find the other new players, no doubt.