I don't understand how others have changed your tree -- unless you gave them permission to. Here is the privacy policy for public trees, straight from Ancestry.com
Public Tree
Making your tree public benefits you and the community
* When your trees are publicly viewable, it is easier to find relatives using the Ancestry.com sites.
* Members of Ancestry.com sites can get in touch with you and help with your research. (This is done without showing your name or email address.)
All personal information is protected
* Information about living people is automatically hidden from others and is NOT included in our search index.
* You can change your mind and make your tree private anytime.
* Only you can edit your tree unless you invite people to work with you.