I think the best solution would be an ignore list, and a feature that made sure nobody on your ignore list would be selected in multiplayer games. Having stuff in the profile is well and good, but we don't see that when joining a MP game. We just see a name and a tier basically.
Or if this feature is too difficult to implement (or not worth the effort from WGT's POV), at least add an icon or something to your ignores, so that when somebody on your ignore list is added to a MP game, you can see he's on the ignore list. That way you can easily drop him or the game, without resorting to checking player profiles or your personal text-file with probably an awful long list of names.