Use the "Delete a Backup Now" feature in Reload
1. Get into Reload Web Administration
2. Click to select a post office profile for which you want to delete backups
3. Select the Backups tab
4. Select the Delete a Backup Now panel
5. Click on Select a Backup to Delete Immediately
6. Select a backup to delete (perhaps the oldest one for example), and then click the Delete Backup Now! button
7. The request will be fulfilled immediately, but the actual deletion process could take a while based on how quickly the data can actually be deleted from the server.