For a pledge being charged as a recurring gift through Luminate Online, cancel the gift in Luminate Online. This will change the pledge status to Written Off.

For a pledge being paid by cash or check and then recorded in LCRM:

Change the pledge status to Written Off. This will not affect installments already paid. Then delete the pending installments.
To change the status, go to the pledge, find the Pledge Status field, double click, change status, and save.
To delete the pending installments, go to the Recurring Gifts section of the page and click the Delete Pending Installments button, then click OK below it. The page should refresh and the Installments section should show only received installments.