Whether or not you can cancel your payment for a Trade Assurance order depends on the status of the order.
1) For orders where the payment is still processing you cannot cancel your payment. You need to wait until the payment has been received by the seller.
2) For orders that are waiting for the supplier to ship, you can apply for refund by filing a dispute.. If you cannot resolve the situation with your supplier then the Alibaba.com dispute team will intervene to help after case escalation.