You can't block the banning via admin menu without modifying the sourcecode of sourcebans.
This plugin could handle the command banning or some other kind of ban via a different plugin, but since sourcebans adds the bans to it's database simultaneously to calling BanClient, blocking the OnBanClient forward doesn't help here. Sourcebans always bans for 5 minutes locally, so one can't check the time in that callback anyways.
Put the bantimelimit.cfg in your sourcemod/configs folder. I didn't test it, but as said, you'd need to modify sourcebans intially to limit the ban times properly
