Step 3: How to block a RFID tag
The signal sent out by a RFID tag is easily blocked by metal. This means that placing the RFID tag inside of a Faraday cage will prevent the information from being read.
There are already two Instructables on how to build RFID blocking containers:
RFID Secure Wallet
Make a RFID Shielding Pouch Out of Trash
Or if you would rather spend money on something you could build, head over to Think Geek for their RFID blocking wallet and RFID blocking Passport Holder.