All you have to do is kill one of them and hang it in a tree nearby. The rest of them will leave and never return.
I hung a crow in my pecan tree one year. No crows that year. To my surprise no crows the next year even though the dead crow had been thrown away the year before.
It seems as long as there is one crow still alive that saw the dead crow hanging from the tree he will warn others to stay away. Once all die off and you have a new generation of crows you need to kill and hang another one.