When all sub tasks of a card reach the last column on a board, Jira offers to mark the parent task as done too. This is a problem as our workflow actually has work associated with the parent card.

This is NOT due to a post function and is a built-in functionality of boards that can't be configured anywhere except by adding a blank column as the last column of the board. This disables the prompt but shouldn't be the way to do it. Making the prompt an optional setting would be ideal.