Log into the management portal and go to the computer account you want to delete stray files for.
Go to the Delete tab and click on the button for Delete Data.
In the window that appears, choose Files and Folders as the backup set type, then Stray Files from the right side. Hit Next to continue.
On the next page, choose which stray files and folders you want to delete by placing a check in the box next to those items. Click Next when you have selected the files you want.
The last page will give you a summary of the files you chose to delete. Hit the Delete button to start the delete.
Instead of deleting stray files manually, you can set up a rule that will delete stray files after they been stray for a certain amount of time. To have this happen, you will need to create a Stray File Retention rule; directions for setting that up are here.