While it isn't possible to fully remove the prescription from the Prescription drug history, you can make it into a red entry so show that it was removed/voided.
To do this go to the patient's clipboard, Then click on the History button. In the history list you should be able to see the entry for the Prescription being printed. look for the entry on the date it was prescribed and the description will read as "Standard Prescription: 'Drug used' " (or which ever prescription form you use).
Once selected click on correct entry on the middle right and choose the option to remove. This will remove it from the history, and turn the entry on the RX history window red.