I am an artist and have made my living from selling my works for the last 30 years.
That said, I agree, if reluctantly, that once the work has passed from my hands it is up to the new owner to do what she wishes with it. (Diana, at least she used the wreath and didn't just chuck it away!)
There's no accounting for taste, which is why not all of my paintings sell as soon as they are displayed. I put a price on them that I feel will reward me for my skill and labor, and once out of my hands they are gone.
A person could own a great masterwork and put a match to it if she wished. That might be a cultural crime, and unethical in the extreme, but as long as we live in a society that values private ownership there's nothing anyone else can do about it.
Paintings have been cut down in the past for whatever reasons...maybe to fit into a different wall space, or to go into a different frame. It happens.
Copyright laws are something else, but I think they involve making prints of another artist's work and selling them for a profit.