I solved it by myself. Its a simple but time-consuming und risky way:
Simply change the permissions of the folder you want to hide from a specific user-role, for instance "customer". To achieve this, select the Folder in "Share" > "Document Library" and use "Manage Permissions" to give e.g. the consumer "no-priviliges". This hides the folder for "consumer". But don't forget a folder you want to hide …
As is said, its not very handy if you have more then a couple of folders which have to be edited. In a daily work-process it's preprogrammed that one forgets to set the permissions for a new folder.
Has anyone an idea, how i can solve this workaround within a script or sthelse? Like: "If i create a new Folder/Space, check if its name is [HIDEMEFOLDER], if true, set consumer priviliges to 'no-priviliges' ".