I believe the reason why those folks succeed is not the fact that they are in a clan, but the fact that they communicate very well and use a good strategy, and that is something that can happen regardless of clan tags. You cannot really disqualify people from playing in a beginner match just because they do what the game is about, that is, good teamwork, even though I understand that it can be frustrating for the other side of the lobby. Also, not everyone who is in a clan will always play with their clanmates, so banning people with clan tags from every beginner lobby would make playing and levelling up quite difficult for them, since they could never play in lobbies meant for their levels.
Try encouraging good communication on your team as well, for example, tell the captains how important it is to discuss strategy instead of just readying up mindlessly. Maybe they will listen if they realize that the team gets better. If it doesn't work, maybe try searching for another lobby where teammates understand the importance of communication.
Also, you will find such unbalanced matches on higher levels too, where one team will constantly wreck the other side, so it has nothing to do with being in a beginner lobby.