Sure, edit your data/ULib/groups.txt admin allow table to remove all the access that is there, then place it in another custom group you consider higher than admin or place it in default superadmin.
That has a HUGE disadvantage though.
Editing ULX's "admin" group, and giving players admin group access just for DarkRP... you've given every other STool or script made that uses "IsAdmin" to test for access the right to use the tool/script.
IsAdmin is a default Gmod player access test. Admin is admin, no matter what ULX rights are given to "admin" group... many other scripts use the same test just like DarkRP does.
Imagine the horror of every admin player having access to prop protect settings, manhack guns, to name but a very view.