Any question answered...

Watermark and DPI. Save sample files in low DPI so any work they do on the photos will almost always make things worse.
Why not just send them samples of already Photoshopped work? (If feasible) That way, any further work they do on it will be glaringly obvious.

He does, he sends them the final versions as what he would use for himself, but some models seem to want to add their own takes on the matter...