Histories and/or datasets must be permanently deleted (not just deleted) in order to free up disk space.
To remove individual datasets while in a history, use the History menu option "Include Deleted Datasets" to view those in delete and permanently deleted status. You can use the links provided to permanently delete individual datasets, the "Operations on multiple datasets" (box with checkmark at the top of a history) to just permanently delete the selected datasets in batch, or the option "Purge deleted datasets" to remove all in "delete" status at once. You may also wish to examine Hidden datasets (using the History menu option to include them), if you use workflows, to see if there are any in that status that you wish to permanently delete.
To remove histories, use the History menu option "Saved Histories", then click on Advanced search, followed by choosing status = all. This will bring up a view of all histories in an account (there may be multiple pages, see the bottom of the form). Status is in the far right column. For any that are "deleted" but not permanently deleted, you can use the menu for individual histories to permanently delete them, or use the check box next to each to select those to purge then click on the "Permanently delete" button at the bottom of the form.
A final location to search for account data is in the History option "Histories Shared with me". Unshare those no longer needed to recover additional disk space.
More details are in this wiki, including a demonstration video:
Hopefully this helps, Jen, Galaxy team