Hrm, at that point, I think you have to find out the Steam ID, I tried looking in my own ESF folder (since it also doesn't use amx) and I couldn't find a ban list. Of course I've never banned anyone with my home server, so it's more than likely that it just didn't create one. My dedicated server has the banned.cfg file in the main folder. Either way, if the person you have banned wants to get in, then have him tell you his ID, and just simply unban it. As far as i know, there is no command that unbans everyone.
#Edit#
Read up a little online and found this
To list bans by WONID, use the "rcon listid" command:
rcon listid
Try that out.