If you were invited to a board but are not participating anymore and would like to leave it, you should take the following steps.
1. Find the board in question on your Dashboard.
2. Open the board context menu and choose Leave a Board.
You can leave a board from the dashboard
3. You will see a warning message saying that this action cannot be undone.
A warning message
4. After you click OK you will lose access to the board and will no longer be able to edit it.
If the board is shared with a public link, you might still be able to view and/or comment on it.