User who has administrator role can do anything on UMP.
Document ID : KB000012616
Last Modified Date : 14/02/2018
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Question:

User who has administrator role can modify page on UMP even though "UMP Read-Only" theme is assigned.

Is this expected behavior?
Environment:
Any
Answer:

Yes, user who has administrator role can do anything on UMP.

There is no way not to permit page management, except removing the administrator role.