Velvet actually does have part of its life cycle in which it is free swimming, so it can live without a host in the tank but really not for very long.
Simply waiting 5 or 6 days with the tank empty of fish and in total darkness (cover it with a blanket or towel if you can) would be long enough for it to have all died. The free-swimming and reproduction stages of the parasite only last 2 days each. It would help to raise the temperature, which will cause the life cycle to go even faster. Once they cannot find a host, the parasites will die.
If you want to manually disinfect it, the parasite that causes velvet is sensitive to salt, so you could use a serious salt solution to soak all the decor in and then rinse it very thoroughly.