Hi @Hannes Stauss ,
If you are the creator of the file, you cannot prevent creators from performing actions on their own documents. The creator has a separate set of permissions, not affected by the protection level/sensitivity label on the document (including printing), as detailed here: https://learn.microsoft.com/en-us/azure/information-protection/configure-usage-rights#rights-management-issuer-and-rights-management-owner
To add additional restrictions to printing in the browser, you could supplement by using Endpoint DLP.
https://learn.microsoft.com/en-us/purview/dlp-configure-endpoint-settings