Go to the ACP - users and groups tab - manage groups - there go to members. Remove them all from groups.
Go to permissions tab - User roles - view assigned items: shows you who uses this role - so you can go and remove it. With this knowledge go back to groups and remove the roles...
That should be a good beginning. If needed you can set all permissions in roles to never once - and sort and set it new.
Assign roles to groups (never to people!), and move members to this groups, when you have sorted your mess.
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My 2 cents: Whether an extension is in the CDB says nothing about its quality. It is more important to read the support topics for it. Better to avoid authors who do not answer support questions themselves, who do not update their stuff, and who do not fix bugs for years.