Under XGUI in the ULX menu you can restrict ranks from using that menu that allows them to set themselves to a higher rank. Just revoke 'xgui_managegroups'. I'm actually not sure if this restricts them from setting themselves to those ranks through the Cmds menu, but you also just revoke the ranks you want from using the command that allows them to set their rank and other players ranks, which is under the User Management tab. As Kewler said, I'm not sure you can restrict people from setting themselves to higher ranks unless you take away the commands that let them mess around and set ranks, for now.