Layer the basement with sheet cotton wool. Failing that I hear sheep actually absorb smell. It's to do with their intensifying fibre jacket. Any smell that's worse than them will be challenged by their skin glands and hence they literally absorb smell. It works better than just wool. It may sound wacky, but we tried it once last year. Although a lot more sheep left our basement than went in after 6 months, it did the job. Hope I've helped.