Ok, best is always to delete a (the) records in banned list table. Sometimes it would be enough to delete only one record (if you know the ip(4)) but hard iff ip6 form.
If it still does not work try to look in table online iff still present (disable if so by changing 1 to 0.
Next could be (doubt) admin log holds some info... delete some (or all?).
That all is for sql. On (file)server you could check if some logs are still present.
Again still no result? than the only thing left is possible a corrupt cookie setting. Clear cache and cookies from browser..
Hope one of those is it.