Are you VAC banned? This has nothing to do with the game, just for the record. It is 100% you, your firewall, your router, whatever.
If you join a server that has ping limiters they'll boot you from the server if you are > that ping limiter (also rate limit boots). I don't care how fast your connection says it is, I am not ruling this out.
Also, please post the message that pops up when you get booted back to the main screen. That is a VERY important message to help diagnose the problem.