Thumbs up to the above!
Prior warning though, if you have lots of groups, then it's gonna take a while . Essentially, you'll need to create a new Limited Administrator account, put a tick beside the "Manage Groups" option under the "Access Rights" tab, then go through every group and switch the permissions to "Read-only" access (even for inheriting groups).
Incidentally, with the "Manage Groups" option enabled, you also have the option to allow the Liminited Admin to run specific commands (if you so wish), even on Read Only groups