Also why not stop first contact by destroying the Vulcan ships?
Uhhh, they know that First Contact is the pivotal moment that leads to an end to war, poverty, hunger etc. for humanity. Why would you want to stop it?
(Alternately, if they went back in time another 60 years or so, and leveled Bozeman, MT, I wouldn't have met my ex-wife. Now that would be an academy award movie in my book.)