Free plan: The archive is a paid plan feature, so the steps below apply to paid plans. On the free plan there is no archive - the project actions menu offers Delete All Tasks and Delete Done Tasks directly on the board, and both delete permanently. You can start free and upgrade whenever you need an archive.

To delete all archived tasks from a project, follow these steps:
Navigate to the Archive: First, ensure that all tasks are archived, as tasks in the main board cannot be directly deleted. Once archived, navigate to the Archive view.
Open the Project Actions Menu: Next to the project from which you want to delete tasks, click on the three dots (...) to open the project actions modal.
Delete All Archived Tasks: In the drop-down menu, select Delete All Tasks. This will delete all the archived tasks from that project.
Please note, deleting tasks is permanent and cannot be undone. Ensure you no longer need these tasks before proceeding with deletion.