Otherwise, they are nothing more than an extremely overpowered wizard who has their powers every round, can extend their powers to preventing others from playing entirely, and nobody wants to play with as a result.
This is exactly what I've heard about BayStation12, hurr durr.
Is it? If so, then perhaps some of the BS12 admins need to improve in some areas, too. Unless the complaints were because someone joined into an admin round that had been openly discussed and approved before it started, and the new joiner didn't know so and thus assumed that it was just an admin jumping in and messing with the game because they felt like it.
Otherwise, I suspect that the ideal thing to do would be to go to the forum and point it out that what they were doing was disrupting the round unnecessarily without a good cause. If they have a tendency to do so often, there might even be an active discussion of it in admin-only forum areas, and it might already be decided that they probably need to be demoted as a result (Note: didn't look it up, just making a generalization of something that might happen. If something like this has happened or is happening at the moment, I am not making a reference to it intentionally).
The other thing is that openly stating it on the forum, where it will visible (unlike in IRC or in-game, where it will only last if someone keeps a log) makes it clear that the players have problems with the way some admins are playing, possibly even prompting a long inter-admin discussion about the subject, potentially leading to new guidelines and perhaps concretely defining what to do/not do in certain circumstances, so that those kinds of problems don't occur in the future.
Agh, I believe I just wrote a long rambly tangent. Ignore it if it doesn't make sense.