As far as I can see, you either put your tree on GR, Ancestry etc and assume it is fair game for people to take/add to their own trees, or just do not put the tree online at all. Another option is to put it up on your own site, in your own format, with your own security controls.
Personally I cannot see why people use Ancestry if they do not want the data taken. It is obvious that it is a hotbed of name grabbers; also that a huge amount of the tree data is rubbish and/or unsourced so useless to serious family historians. I just need to see the odd distant relatives of ours who are on various trees. 90% wrong! They have looked at one census, and made assumptions. My husband has a rare surname, and it was often mistranscribed - but 90% Ancestry trees with this name have ignored this possibility and not searched properly.
I would never post photos there - why would you, if you do not want them to be in the public domain?
I keep all my info on my own computer. I have had three cousins of various degrees contact me via rootschat - I am happy to send data I have on the relevant rellies. But really I want to prove all of myself, with proper souces.