If you want to tidy up your boards dashboard without permanently deleting your boards, you can archive them instead. This puts them in an archived folder away from your other boards.
Archive a board
Go to the Board Options menu ("..." icon top right of your board)
Select "Archive board"
Click the ‘…’ icon below the board you want to archive
Select the bin icon
Choose ‘Archive’ when the dialogue box pops up
Restore or permanently delete a board
Click ‘All Boards’ at the left side of your boards dashboard
Choose either ‘Restore’ or ‘Delete’