It's simply impossible to prevent the AI from attacking you on Hard or Very Hard difficulty levels. The horrible diplomacy of the game insures that. There are, however, a few things you can do:

-Make alliances with your neighbors, and give them smallish amounts of mnai- around 1000- to each of them each turn to keep them happy.
-Keep your borders well fortified. Strong armies discourage the AI from attacking.
-Do what you can to stir up other wars between other factions so they're too busy to attack you.
-Use the Force Diplomacy minimod to get ceasefires.