How do you delete missionaries from the drop-down menu under donation?

We have a few returned missionaries who have been home for several months but whose names are still showing up in the missionary donation drop down menu. They of course are not currently missionaries.

How do we delete their names from the drop-down menu?

First, you have to zero out their account.

Normally, they will disappear after a few months of zero balance, but you may want to call support and see if they can delete it sooner to prevent a well-meaning person from donating and resetting the clock.