When you delete a recurring task, Marvin can ask whether you want to delete only this instance or all the instances. If you don't want to see this question every time, you can turn it off in the recurring feature settings.
Turn off the delete question
Open the features page and find the Recurring feature (search is easiest) ( Press "S" on your keyboard).
Click the recurring feature card to see its settings.
Turn off the option that asks whether to delete this instance or all instances.
From now on, deleting a recurring task no longer shows this question.