See the space permission documentation at https://confluence.atlassian.com/display/DOC/Assigning+Space+Permissions. Then configure your readonly user in the Individual User area to only have view permissions for the desirable spaces.
Don't give confluence-users (that is the default group that all users belong to) editing rights in any space – use space permissions to only give viewing rights.
Create a second group (confluence-editors, for example) and give that group editing rights in each space. Add the users to that group as they earn the rights.
I think that is as easy as it is going to get.
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