I haven't yet banned anyone for wanting to play on the stronger team. I might not think great of them or I'd give them a filthy look, but I wouldn't force them to play in the opposite team. There are other 15 players which could be transferred and 32 to be swapped.
Balancing teams is important that's why every admin should do it first hand, specially if they are skilled, they should balance teams by joining themselves to the losing team. If that doesn't help swap 2 people from opposite teams to get the best results.
I always self myself to losing team and I would ask every admin to do the same. I have seen this plenty of times on both, public and DD2, some admins don't care. You should care.
Another thing is you might be right to do so and you are legally allowed, but should you? As much as I don't like unbalanced teams, I find this rather bizarre, to ban players for wanting to be on the stronger team.