JohnRoy - 5:26 am on Mar 1, 2011 (gmt 0)
For issue 1) have them click on some sort of I Agree check-box before submitting the order - clearly reminding them about your return policy. It might help later when disputing, and even if not, it might help keep this attitude guys away from placing the order.
For 2) calculate the costs. Some merchant services charge up to 50 bucks for each charge-back, regardless of who wins the dispute.
3 in one week?! - that's awful :(