An option on the Management Menu will purge deleted data, in order to use this option System Administrator permissions are required.  Currently this option will only Purge Result data.

When Results are deleted, they are not actually removed from the database, the rows are simply marked as ‘Deleted’ so that they no longer appear in any grids. This however does not enable the disk space to be re-used. Therefore this option will actually remove any result data marked as deleted from the database.