I may be misunderstanding what is being talked about or this may have already been said, so if I am, sorry.
If you look in the admincmd plugin, you will notice that the banning is being done like this (and this is how I would rather do the banning instead of using the amx_ban command)
kick #43 "You are banned";wait;banid 0 STEAM_0:0

2151;wait;writeid
That is the true way to ban them. If you just use banid, the user will still be in the server, you have to kick them to get them out, banid does not do it for you (unless you put the 3rd parameter as "kick", but then you don't get to display a kick reason).
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