I received quite a few requests to delete diffs that accidentally shared, so I made this delete permanently feature recently. Yes, it means anyone with the link can delete it, so don't give it to anyone you wouldn't trust, but I wouldn't say it's "impossible" to share stuff publicly. mergely.com
currently has no user/login mechanism, or even a backing database, so it isn't really currently possible to permanently track the author of diffs. I would suggest that the share mechanism would work just fine, sent to individuals or teams. But if what you say is true, that people are deleting diffs nefariously, then I will have to do something about that.