Ok, to be perfectly honest, I've never actually had to do what your trying.
Your server may explode. I personally don't recommend mis-use of remove/add user. It's superadmin for a reason by default, even with our uber awesome restriction system.
BUT, from what I see here -
http://forums.ulyssesmod.net/index.php/topic,4872.msg20767.html#msg20767(Not a faq, but knew there were 'how to limit' discussions somewhere, you
MAY be able to limit your s.admin group with something like.
ulx groupallow s.admin "ulx adduser" "* !%staff"
Same for ulx removeuser.
If you have more than one higher group than staff, use commas.
ulx groupallow s.admin "ulx adduser" "* !%staff,!%<higher_group2>,!%<higher_group3>"
You'll have to figure out on your own how to prevent your s.admin from adjusting staff level members. Might be *,!%staff in that first parameter.
Now, all the above being said, we really, and I mean REALLY, don't advise giving anyone adduser/remove user access in this way even if it works.
Those commands are meant for root/superuser only. It's why Gmod created it that way.
I understand the whole convenience thing, but, really, trust should be seriously earned.