The titles says all! When a task is ticked, the reminder might stay as it was set but the notification should not appear anymore. It is a hassle to remove the reminder on top of ticking the task, and confusing to get a notification for a task already done. An alternative would be to delete the task but this is not convenient as it does not keep track of what was done plus if there is an update shortly after task was done it is convenient to keep the task marked as done as a template. In summary: please simply disable the notification of tasks marked as done (and keep (or not) how the reminder was set for this task).