Bannable Names

There are hard restrictions on what characters you can use in your name, and you can find the regional list here. The new client may still be bugged and accepting other characters, but you won't be banned over it; if you choose a name that is offensive or otherwise violating guidelines you may be forced to change it in the future, so if I were you I'd just stick to the characters on your region's list and not pick anything offensive.

You'll never get banned for a name, they will just force you to change the Summoner name in question.