The goal of this type of attack is to overpower the RF field so that communication
with RFID tags in close proximity is rendered unusable. If an attacker uses enough
power, she or he may be able to disable communications with all RFID tags in an entire
warehouse.
This attack may have severe effects on RFID business applications. An example of
this would be a signal jamming attack carried out against a large retail location. The
retailer could be using RFID tags to track shipments and inventory and may also be
storing pricing and other information on the tags. A successful attacker, using an RFID
jamming device and an illegally overpowered antenna, who interfered with this process
would make data collected by the RFID system unreliable, forcing the location to return
to a pencil and paper system to confirm its inventory.

Preventing Signal Jamming Attacks

Follow normal physical security procedures. This type of attack will prove to be very
hard to detect without physically inspecting the surrounding area regularly. A signal
jamming attack may use a hidden, timer-activated portable device, so security personnel
must be trained to look for changes in their environment, not just the presence of
unauthorized individuals.
All changes in location or status of a tagged object should also be confirmed by both
parties involved in the change. If a wholesaler delivers a product to a retailer, then the
amount of product transferred is recorded on an invoice. If either party notes a
discrepancy, investigate it. This is an obvious tactic when dealing with an outside vendor
because payment will be made for the goods delivered, but this tactic should also be
used internally. If an item is moved from the back room to the floor, confirm the move
with a change in inventory for both locations.(from wikinut)