Is it safe to eat freezer food with bad odours?
Food placed in a freezer with bad smells will definitely absorb odors, unless it's in very very airtight containers/packages. It won't make it unsafe to eat, just very unpleasant.

As Michael said in his comment, the odor is likely in the plastic now, so it's going to be pretty hard to get rid of. You'd want to clean every last trace that you can - use brushes for hard to reach places, use whatever cleaning products you need to help dissolve caked on stuff. After that, all you can really do is let it air out until it doesn't smell too bad - door open, preferably some moving air (outside would be best). It's hard to say exactly how long it will take or how successful it'll be; that depends on all the details. Depending on how porous the plastic his, how nasty whatever soaked into it is, and how long it had to soak in, things could vary widely.