Step 1 - Go to Settings
Click on the settings icon in the top right of your app.
Step 2 - Go to Your Accounts
Click on Your Accounts and select the board that you want to delete
Step 3 - Press and hold to delete items
Now press on the item you want to delete and keep pressing until you see a "X" icon appear to delete the item. It works the same way as deleting apps from your device.