Click the "My Forums" cube.
Click on the "Manage Forums" button.
Click on the forum you want to delete.
If you are the administrator of the forum, you can click the "Delete" button. If there is no Delete button, but there is a "Unsubscribe" button? Then clic that.
Click the "Delete" button again.
Please note! Everyone who you have shared the forum with, loses it. They will get a notification. The participants of the deleted forum will receive a message.