Sorry for the trouble. The error you reported sounds like a software bug. But I just tried the ban feature and it works fine with no errors. I will forward this as a bug report to the programmer, but in the meantime, can you try again and see if you can reproduce the error?
We looked into it, it turns out to be a harmless message. The message shows when the user has already been banned. So, you can ignore that message for now. In our next release, we'll remove this error message.