I've started banning people with higher than 10:1 KD for being too good, especially if I don't know them, and their Steam account is private.
If I recognize the name from the beta or tournaments or something, or if they have 500+ hours of NS2 on their Steam account (or something like that in CS or something serious which indicates that they COULD be that good), that's fine.
If it's a 30:3 KDR, private account, especially with a different name, or has VAC bans with 20 hours of NS2, etc., or very few games or only NS2 or something with low hours - bye bye.
And yes, I've actually seen the server die because of the stacking which also occurs around such players - hell, even without stacking one player which has the combined kills of his entire team is ridiculous so you have to stack AGAINST them to have an EVEN-ish game. I'm guilty myself - although it's probably because I like playing Marines more lately, and I've noticed those "types" of people tend to favor Marines as well... but yes, it's a problem. I don't want to see random perma-on, because frankly random can suck my dick... but maybe that's just me.