Aiptasia and majano anemones can be in spots where you just can't reach or get too. Leave just one and the population returns. The only way to rid the tank and rocks completely is to dry everything out for a few days, run fresh water for a few, then rinse and start over. Skimmer, sand, filters, reactors. Everything that was in that sytem that you plan to use again. Or you could just run freshwater without the drying and rinsing but not as clean an approach.
Could also "cook" the rocks in saltwater, in the dark, with water flow for a few months. Where everything except maybe some copepods will survive. The tank, filter, skimmer and anything else that was in the system will still need one of the above freshwater approaches.