To prevent concurrent modifications, Structurizr will place a lock on the workspace when the workspace/diagram editor is opened, and will remain for 2-3 minutes after the editor is closed. Other users will not be able to open the workspace/diagram editor while the workspace is locked. If needed, locks can be cleared immediately using the "unlock" link on the workspace summary page.
The ability to lock a workspace via the API (e.g. when using one of the client libraries) is coming soon.