Cut the straps so that the seat can't be used again. Write on it in big black permanent marker that it's expired and can't be used. See if you can get a discount in an exchange for a new one. If not then you need to throw it away.
It's not a waste if they're thrown out. They literally MUST NOT be used.
To KiKi...
How Car Seats Expire per the National Highway Traffic and Safety Administration (NHTSA):
*Technology Improves and Standards Change. A good example of this is that back in 2002 car seats were not even equipped with Lower Anchor and Tethers for Children (LATCH). Now however, they are a standard feature in nearly all car seats. Car seat expiration dates ensure that seats being used are current and up to snuff.
*Materials Wear Down. Car seats were not made to last forever. Over time the seat base can develop hairline fractures which may shatter in a crash (Should you replace your car seat?), and belts can become slightly elastic after years of use. Crash test videos provide a chilling example of how these materials can fail in a devastating manner.
*Only Tested for a Certain Period. After a certain amount of time, manufacturers do not test seats. They cannot attest to how older seats will perform in an accident.
http://baby.about.com/od/healthandsafety/f/car_seat_expir...