Try 'site help bans' for details, but 'site bans remove <ip>' should do it. Note that is just the IP and not a full hostmask. You can use 'site bans list' to see the current bans which should confirm it's been removed. You can use wildcards when specifying the IP so 'site bans remove *' will remove all temporary auto-bans.
If none of that works please cut/paste the exact text (obscure the IP) of the bans message. I just want to make sure it's an auto-ban and not any of the various other types.