Mia it's not just clan that 'distort the team DM gamemode'. Many classes individuals also make alliances or do selective killing.
I agree that swiping clans might be a big step to restore the PTP gamemode BUT it would definitely defeat the purpose of playing this old multiplayer game at first place(for many of us), that is to communicate and have fun. Not saying that a clanless person won't have anyone to talk to or won't enjoy much but being in a clan is fun and a LOT more than just "hunting newbies together".
-1. You can try giving up supporting clans officially, but in the end the clan will still hold and the problem of "alliance" u were trying to solve will stay as it is. Not allowing the clan tags completely, on the other hand, might solve this to an extent but players will feel hollow and definitely move to a better game or something (unless you are a no lifer with no better things to do than forcefully playing a game that u find boring) with the clan members.