Really the only sure way to get rid of these pests is to give in and use a permethrin spray. This is a relatively low-toxicity chemical (used also as an ointment on skin to kill scabies etc) I've tried all the soapy stuff and the things come back laughing. Pyrethrum and permethrin are very similar to the natural insecticide found in marigolds an other daisy-like flowers. A small aerosol or plastic hand-sprayer is the easiest way.