Can I mass delete data other than through import or REST?

No, you can't delete data in bulk except through import or REST. However, when you log a service request to get your environment cloned, you can request it be cloned without the data.

The exception to being able to delete using Import Management or REST web services is that Account object data can't be deleted. It can only be inactivated or purged. See Can I purge inactive account and contact data?.