Lock Client Options for users but not for admins?


I just want to make sure that I don't miss something:

I want lock a setting (Cleanup, Empty Trash, Automatic After XY days) for
all users, so that they can't change it.
I want XY for most users the same, but for some different.
I want to set XY on domain or post office level and change this on the user
level for some users.

This is not possible, right? Locking a setting at dom or po level not only
prevents users from changing this, but also the admin?

Probably I need a separate Post Office for doing this?