i have tried to kill ones in my tank and i have done it to them over 8 times and they are still alive, one trick a person told me is taking boiling hot lemon water and with a serenge inject into the body of it. i guess it can work but the thing about these little annenomys is that they have little eggs of more babys in there stalk and killing them that way can relise the eggs into the water stream. the reason that aptasia x does not do this is because it is highly concentrated kalkwasser and if it killed my aptasias, wich i wish it did, it would kill the eggs as well, when the boiling hot lemon juice may not kill them.
cottonwood probably has the best idea about the nudibranches, they should take care of aptasias, but that is all they eat, they eat nothing else, so after all the aptesias are gone then they will die, so the best thing to do would be to give them to youre lfs so they can fix someone elses problem :)