That was what I thought you wanted to do, since you pointed out the fact that they could add, but not remove users. By default the group "superadmin" has all access, so I just assumed that you wanted them to be able to remove users too.
In that case you would have to use these commands:
ulx groupdeny superadmin "ulx adduser"
ulx groupdeny superadmin "ulx adduserid"
This can also be done through RCON console, but I would actually recommend going through the ULX menu and unchecking/checking the commands that the group "superadmin" has access to through that. It would be the best if you want to limit what the group "superadmin" can do. Although, if you're creating some sort of "owner" group, the way that you're doing it is not recommended (
read this). It's better to make yourself superadmin with all access and make a group that inherits from "admin" and then you can add certain commands to the new group.