The solution to stop your auction immediately before it ends is to disable your items. To disable an item, you can change the status of your Product from Enabled to Disabled. Go to Catalog->Manage Products then click on Edit, and under the General tab, scroll down and adjust the Status from Enabled to Disabled. Click to Save the item. That will hide your auction item from bidders so bids can't be placed and any bids already placed on the item won't be processed.
To adjust the status of ALL of your items from Enabled to Disabled in bulk, you can follow the steps found in this article: https://winningcause.zendesk.com/hc/en-us/articles/209804508