I was going through the Clients link on the MR AP page. It wouldn't let me block them completely though, it said they were blocked, but they were still using bandwidth. I was able to reduce their bandwidth as much as possible (down to 20kbps), which I think annoyed them enough to stop doing it, but they were not blocked.
They have left the campground, so the issue is gone now, but I want to be able to have the option to do it in the future.
We do have an MX and three MR. I was going through the MR page though, didn't even think to go through the MX page, but I'll try that next time we have an issue.