Yes, if the order has not been finalized yet (it is still active). If you wish to cancel your order/purchase go to "Orders", then you can either click on Cancel all or if you wish to cancel an individual order, click on the arrow next to the order you wish to cancel:
Click on Cancel Order to finalize your cancellation:
The status of your order will be changed from "Active" to "Cancelled":
All confirmed transactions of any kind on Blocktrade.com platform are final and irreversible.