Insecticidal soap will kill them but you have to spray VERY thoroughly and you have to commit to 3 thorough sprayings a week apart.  The insecticidal soap spray will kill the adult insects but no the eggs that have left behind.  The second spraying is to kill the insects that hatched after you sprayed and, hopefully, before thy also lay eggs.  The third spraying is to insure that you haven’t missed any with the first two sprayings.  If the sprayings aren’t thorough enough or too long in between, you’ll never be rid of them.  When you spray, you nee to make sure the spray has coated the entire plant…every square inch.  Many times people miss the undersides of the of the leaves.