It works in sonarqube 6.x, but no activate/deactivate button found in SQ 7.2.1.
The default quality profiles are indeed no longer editable. This is noted in the upgrade notes for 6.5
You can copy a default quality profile and start make changes, such as deactivating rules. This can actually be preferable as many organizations want to tightly control or at least communicate changes to quality profiles, instead of being caught by surprise on the first analysis after an analyzer upgrade.