You will need the Full Write Pledges permission to perform this action.
Go to "Contributions > Pledges > Listing" from the left hand navigation menu and locate the campaign you want to delete from the list. Then, click on the arrow in the dropdown at the end of the row and select "Delete". A confirmation message will appear. Click Yes to finish deleting the campaign.
You can also delete the campaign from the details screen by selecting "Delete" from the "Actions" menu in the upper right of the screen.
Deleting a campaign will NOT delete any contributions credited to the campaign. Only pledges and the campaign details itself will be deleted.