Look at the space permissions under "space settings" to see how they are getting admin - you'll need to look at all the lines with a green tick mark in the rightmost column.
Once you know how they're getting the admin rights, you can work out how to remove it from all spaces.
I suspect it's probably a group, and one that is named in the default permissions (a global admin setting) for new spaces, so I'd check that as early as possible as you'll probably want to get rid of it - that won't change existing spaces, but it'll stop new ones picking up unwanted defaults.
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