1) ULX has no way to limit the ban command to different groups. They either have ban access, or they don't.
You could use rcon access as a superadmin and set permanently if you wish after someone is banned 7 days.
Find in /addons/ulx/modules/util.lua the function named "ulx.cc_ban"
Find the following lines
local time = "for " .. bantime .. " minute(s)"
if bantime == 0 then time = "permanently" end
Change them to
local time = "for " .. bantime .. " minute(s)"
if bantime == 0 or 10800 then time, bantime = "one week", 10800 end
and a few lines down from that change
ulx.addToMenu( ulx.ID_MCLIENT, "Ban (perma)", "ulx ban #T 0" )
to
ulx.addToMenu( ulx.ID_MCLIENT, "Ban (one week)", "ulx ban #T 10800" )
2) That message your seeing is most likely from the permissions list in gmod/settings/users.txt, not ULX.
Gmod by default does superadmins and admins. Not sure any other groups are checked by default.
You may test by adding an individual to a newly created group in gmod/settings/users.txt