Hi Julian

Thanks for posting in the Lucidchart Community! The answer to this question depends on if you're using Confluence Cloud or Confluence Server. 

If you are on Confluence Cloud then editing permissions are inherited from Lucidchart. This means that it is possible that users have access to the diagram on Lucidchart and therefore can edit it on Confluence Cloud too regardless of the permissions set up on Confluence. This would explain what you are currently experiencing.

If you are Confluence Server then editing permissions are inherited from the Confluence Page not Lucidchart. This means that if you have restricted users' editing permission to view-only on the Confluence Page then those users should not be able to edit the diagram on Confluence regardless of their Lucidchart permissions. If you are on Confluence Server have restricted the Page's permissions to view-only and users can still edit the diagram then we will need to move this question to a support ticket to further investigate what is going on. Please let me know if this is the case.  

I hope this helps! Feel free to reach out if you have any additional questions or concerns.