power users shouldn't be able to edit administrators' subgroups

As a power user, I can make users administrators by:

• editing the subgroups of the administrators group (from the Groups tab),
• making users members of the administrators' subgroups (from the user's Edit tab).

However, after I added the user to an admin subgroups, I cannot edit the user's information anymore.
I shouldn't be able to add users as administrators even by editing administrators subgroups.