Expire tasks from Kanban?

If I move a bunch of old tasks to a separate kanban list, then delete the list, the tasks will go back into ‘backlog’. Even if I archive a project, its tasks stay in the kanban board. As a result, I have this ridiculously long ‘done’ list of every task I’ve ever completed.

Surely there has to be a way to retire tasks from the kanban, right? But how?

Tasks are automatically removed from the Kanban once they’re invoiced.