The REST API has a _purge command for doing this — it physically deletes document(s) without leaving a trace. Unfortunately, for some reason it hasn’t been implemented yet in SG.
In principle you could safely purge the documents by deleting them via the Couchbase smart-client API. That might end up confusing Sync Gateway, though, so I really don’t want to recommend it … maybe one of the SG engineers can comment on that.