Here is what one site says to do:
1) Discard any charcoal, lava rocks, ceramic briquets, or other porous materials.
2) Fire it up as hot as possible to kill it and burn off all grease and mold. Use a double load of charcoal. If you try to scrape it off or powerwash first you might inhale a lot of spores and have an allergic reaction, and you will still have a lot of mold on the floor.
3) Let it cool and now scrape and scrub everything in sight with a wire brush or a pressure washer. Remove parts and scrape or pressure wash them. The only desirable grease is elbow grease.
4) Then wash everything down with soapy water. Then rinse thoroughly.
5) Finally, fire it up one last time to burn off any mold, grease, and soap residue. You're now ready to cook.
I just flame it good with the weed burner, wire brush, flame again, and wash with a damp rag.