In a certain scenario you won't be able to remove Boards. You will get a following error while trying to accomplish that:
It's an error based on presence of Card templates in your Board. You should remove all previously created Card templates to solve this issue and consequently, to allow yourself to delete a Board.
To remove a Card template, you should enter Settings -> Card Templates in a Board menu.
Click on delete in the Card Templates window.
Customer support service by UserEcho