Take your sand in a bucket, and put in a garden hose as deep as you can. let run while stirring with hands(or turn up the hose and use water pressure) until water runs clear this will flush out all the silt and probably the worms as well. There is no real way to make sure that all bristle worms are gone from your rock, a freshwater dip might cause them to flee their hiding places, but it's not a guarantee. I would probably leave the rock alone, unless there is something else that you wish to destroy. you can pull it out and let it dry, but I've found a blowtorch works very well at killing everything it's pointed at (come back from that, aiptasia.) I've never had an issue with bristleworms, as I rarely see them, except when I feed. i usually feed fairly heavily and they help clean up the mess.