There is already a css style to indicate a quote has been edited, as I recall.
In general this is an issue of moderation, not software features. Similarly, users can post “blank” posts by using obscure Unicode spaces, etc. and there are about one million ways users can grief each other with Unicode, across the breadth of the internet.
Rather than imagining it as a problem in your mind, try seeing if it is an actual problem in practice, in the real world, first.