amx_unban should work:
Code:
amx_unban "STEAM_0:0:1234"
Otherwise:
If it was a temporary ban (a length of time that is non-zero) then you can shut down the server and then start it again. This will remove the temporary ban from memory.
If it was a permanent ban, the ban will be listed when you use the command "listid" in the server's console. If it is listed then you can remove their ban with the two commands:
Code:
removeid STEAM_0:0:1234
writeid
This is how amx_unban works.