Why is the "Ban player" option missing in SourceMod's admin menu?
Most likely SourceMod's basebans.smx was still loaded when you installed SourceBans' game plugin. The game plugin automatically disables basebans.smx, but your server needs to be restarted for the "Ban player" option to come available.
Some hosters might disallow plugins to move files. Make sure the basebans.smx is in the plugins/disabled folder.