How can I remove odor of spilled sewage under the sink?
At pet stores (and many other stores) they sell "enzymatic" cleaners that are made to destroy urine odor.  In my experience, they also work on other stubborn smells that can't be cleaned by ordinary cleaners since they work by actually breaking down the odor causing contaminates.

If that doesn't work, there's also a type of primer that you can purchase at a paint or home improvement store that will help seal in odors.  A shellac-based primer like Zinsser BIN will work well.  I've used it on paneling that reeked of smoke and on sub-floors where an animal died, and it worked well.  It dries very fast, so it's easy to use, but hard to clean up.