You have to remove everything, turn off and scrub down with vinegar and baking soda. Dry, sprinkle a box of baking soda in with door open let it sit a few days. Clean it out. Hope it works.
Fill a pie plate or shallow bowl with baking soda, activated charcoal or cat litter if the decaying smell remains. Set the pie plate in the freezer and turn it on. Run the freezer empty except for the pie plate for two to four days to absorb the rest of the odor.
That is tough to have that smell. There are some good suggestions out there..
Here's a great post on cleaning a deep freezer.