[How do you ban a person from sim, no matter which alt they use?]

I seem to remember that IP banning is not allowed. However, I know that some sims have security that prevent a person from coming back on an alternate account. Is there an allowable way in which I can prevent a person from coming back to our sim, no matter which account they use?

I'm aware that I can close the sim to the public, and have it be group only. I don't want to do that. 

[Why can't group members enter the sim, that is owned by that group?]

I recently switched ownership of our private sim from this avatar, to group owned. I had to create a new group. The old group allowed everyone onto the sim, but it was owned by someone who was no longer part of our group. We could not have the original group own the sim. 

The land was transferred over to be owned by our new group. The settings under access are for our new group. Everything seems to be set up right. 

However, a handful of avatars that are in the group cannot enter the sim. Some of us can. All have the group activated. One avatar was able to be on the sim for hours last night, then suddenly, she could no longer be on sim. 

Any idea what's happening?