You can remove admin permissions for a member of staff in one of two ways.
The first option is to find them in the organisation chart and click on the cog by their name. From here, you can simply untick the 'admin' box and save the change.
Alternatively, head to their profile and click on the cog in the top right corner of their profile header, then select 'remove admin'.
NB: Note that only admin users can change admin permissions. As an admin user, you cannot edit your own permission settings. Another admin user will have to remove these on your behalf.