When you have enable_shared_bucket_access: true in your Sync Gateway config, N1QL deletes will not be purges, they will be tombstoned for replication to mobile clients. These are necessary during normal use, when clients need to know that a document has been deleted. You can read more about this here: Tombstones | Couchbase Docs
If you need to empty the bucket completely, when developing/testing, you can flush the Couchbase Server bucket: Flush a Bucket | Couchbase Docs
If you need to purge documents on an individual basis, you will need to do this through Sync Gateway, via the _purge endpoint, as you’ve seen. But be aware that mobile clients who have pulled these documents will attempt to push the document back up as a new one.