Parisgreen thanks for your reply... yes this works, but it's not what I want. Admins and developers should still be able to use the source-editing feature.
At the Main Configuration file (config.yml) there is already this option set (~ at row 285):
viewSourceRoles: [ 'admin', 'developer' ] # The roles which are allowed to use the [Source]-button.
Maybe someone of the developers (@bob, @gawain, @PhillippOhlandt ) can tell me how to use that option? Is it only working on older Bolt versions? (I do have Version: 3.0.9)