The best way to prevent employees from saying bad things about you is to hire the right people, give them the resources they need to do their work, and let them do it. When problems arise deal with it based on what happened.
When you fire someone, it should be for their actions, not because they violated a policy. If employees are constantly violating policies, you have a serious management problem. And more policies won’t fix it.
I agree wholeheartedly with Heather’s advice, and not just because she cited my two-word blogging policy. Along the same lines, here’s an approach for how companies should handle having a Facebook policy.