Deleting tasks en masse is a chargeable service. This can be run while proclaim users are active within proclaim so requires no 'downtime'.
If you would like a bulk of tasks deleted from Proclaim, you will first need to:
Move all tasks you wish to delete to a dummy user profile
Only move tasks you wish to delete in this user's name
Once done, you will need to raise a new case online and reference the title of this article.