We recently banned a troll from our citadel whilst doing a mass cap event. upon adding him to the ban list he is supposed to be removed from the citadel within 3 minutes i believe?
I'm not exactly how this works, but I do know awhile ago there was a lure involving kicking a player out of the Citadel, so there may be a delay to prevent this from continuing.
Anyway, it may also be due to your clan allowing guests in your Citadel if you have that option toggled on because banning the person would not remove them from your Citadel in that case.
If this issue is currently still going, I would recommend checking to see if your Citadel allows guests to enter, which can only be toggled on/off while in your Citadel. The setting can be found under Clan Settings on the same interface that you can edit your clan's homeworld, timezone, guest access to clan chat, etc.
~ Sicariu
Leader of