Please explain what I would need to do.
What you would need to do would depend on the type of tag it is. If it is a passive ultrahigh-frequency (UHF) RFID tag based on the Electronic Product Code (EPC) standard, then the tag has a kill function. If you know the security code that you need to transmit to the tag, you can send that code and deactivate it. If you don't have the code—or if it is a passive high-frequency (HF) or low-frequency (LF) tag—then the best way to deactivate it is to zap it with electricity.
Often, you can accomplish this simply via the static electricity that is emitted through your fingertip after rubbing your feet on some carpeting, but if you want to be sure, put the tag in a microwave oven or get an old extension cord and cut off the end where you plug in something. Strip the wires bare. Then plug in the cord and carefully touch the two ends to parts of the microchip or antenna. Be sure not to electrocute yourself or burn down your house.
If the tag is an active tag, you can simply remove the battery. Overloading the circuits with electricity will also kill an active tag.
—Mark Roberti, Founder and Editor, RFID Journal