Disable download

I'm not aware of this but I think it's a good Idea, you should submit it for people to upvote : ) 
BUT, if you went down that path, once the proof is final, your proof owner would need to add a new version as a document (not a proof) as the final file to be downloaded. 

It would be a good setting to have in Setup here - https://micron.my.workfront.com/setup/proof/settings

If people are using files prematurely, you could entertain the idea of your users putting a cover page or watermark on assets. Our creative team does this, the cover page of a proof indicates it's not final/it's for review, and when a proof is final, they upload the final version as a document.