You can't remove the contents of a ban from the samp.ban file via command (Until I am proven wrong), only by sending an
RCON Command 'unbanip', where the IP is saved in the user's file, rather than gathering it from samp.ban. Then you need to check if the file has ban set to '1', if not, return a message saying otherwise. Then simply set it to 0, and send the rcon command unbanip. I did have a command that did this, but it's not on this computer and I won't be able to get it until a few more weeks... so :S
Good luck.