Each plan provides you with some SimpleStorage space. It starts as a fixed amount, then can be increased by requesting additional space.
To clean up SimpleStorage, you have the option to purge it. This can be done simply by:
Going to the Storage page https://my.simplebackups.com/storage
Click on the actions next to SimpleStorage
Choose "Purge"
This will clean up your SimpleStorage space by purging all its existing data. It may take a little while to complete.