Looks like a bug, I've just replicated it on ours, nothing happens.
In earlier versions there was an issue where if a user still existed in the database for such things as dashboards or the owner of a rule, until all of those associations are removed you can't remove them from the user list.
I recall having a ticket on this and it requiring several databases within the ESM being searched for the user name and then deleting it. I'll see if I can find it and post the details.
Thanks, i look forward for your news.