dont soak in tap water. The rock will absorb copper if there is any. Just not a good idea, unless thats what your using in the tank. Lemon Juice needs to be injected into the anemone at its base with a small syringe. Its the process that i used and it worked very well, but its not really a method that should be used in smaller systems because of the potential disaster. I feel that your best bet is to re cure the rock after a good soak in non salt mixed water.
You can purchase joes juice, which is designed for this and kill the anemones that way.
As i said earlier though, soaking and recuring the rock will kill all the babies if any were released and anything else on/in the rock. Recuring in a small rubbermaid is easily done.