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 "X" next to the order you wish to cancel:
Clicking the "X" will immediately cancel your order. "Cancel all" will ask you additional confirmation to cancel all active orders.
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.