If he is using USGN/Steam: banusgn or bansteam If he is using the same name all the time banname
If he's neither using USGN/Steam nor the same name you should consider to enable sv_usgnonly on your server to only allow people with USGN login. Afterwards if he comes back you can simply ban his USGN.
@ Mami Tomoe: I dislike re-naming and therefore destroying old settings/functionality. So I would rather add an additional new setting for Steam. Unfortunately there is no space in the create server and server filter UI anymore. So this would require some major re-layouting. This is why I simply did not add this function.
It's easy to simulate a comparable behavior with a simple Lua script though (kick players without Steam ID).