From what I have found searching the online forums the only way it seems possible to remove data is by removing the user that the forms were submitted by. Or by re-assigning forms to a destroy user and then delete that user. The other options is archiving the form, but that doesnt actually delete it.
These forum answers i found are on old forum posts from a few years back. Is this still the way that it operates or is there an easier way to do this?