I think it already is in the rules and I've generally tried to tell sharers to respect the wishes of creators. I have nothing against photographers and my curation projects actively support them (photography lovers community in OCD).

That said, it's hard to maintain and control, if you look at his latest shares it's all unique sharers, they all need to note that they can't share this particular persons content, it's even harder for me to know because I go through so many shares manually each day I can't keep track of who doesn't want their stuff shared when I need to look at so many other things when determining the vote strength towards the share.

We're actively discussing this and any help/solution is appreciated.