Is there a banlist or way to block certain players from being in the games you play in Starcraft 2?
No. There isn't. Especially not on the ladder. Then you could say "Block everyone else in the top 200", and you'd never lose! (if you were good enough to beat everyone else)
According to this post, you can block players and you will no longer be matched with them. This is the equivalent of how WoW's Looking-for-dungeon feature will not group you with people on your ignore list. I'm not sure if this will prevent you from playing against them.