Yes, a label can be deleted at any point before it is downloaded/pre-advised.
As long as the label is not pre-advised or downloaded it can still be deleted. Just use the waste basket icon on the right hand side in the shipment overview.
Once a label is pre-advised/downloaded, this information is sent to the carriers and, after this process it cannot be deleted.
If you’ve noticed you’ve made a mistake, and cannot delete the label then you can request a credit using the function in the shipment overview section of SendMyParcel.com.