When you are finished with an Analysis, you should delete it to free up the space.
From your list of Completed Analysis Runs, the Delete button on the right of the page will, with a prompt to confirm, remove the run from your list and set a Flagged for Deletion tag.
After a period, set by your Administrator, runs which are Flagged for Deletion will be automatically deleted forever.
Before this time, you can request via the Adminstrator for a run to have its Flagged for Deletion tag removed. This will bring it back to visibility in your Completed Analysis Runs report. From the Settings screen, click on Recover Deleted Analysis Runs.
As Administrator, you can manually perform the permanent deletion from the same Recover Deleted Analysis Runs screen via the Delete button.