flea beetle on brassicas - cure/deterrent?

[peanuts]

Never had this problem before, but  have had a fairly severe infestation of flea beetle on my young  kale and broccoli plants, and also Chinese leaves, which are near them.  Sprayed them successfully with "Ultimate bug killer" as there was no sign the next day.  Four days later they are back again, and also already on the sprout plants I planted only yesterday evening! Certainly not using any more insecticide (that tends to be a once a year usage when really necessary). 
Can anyone give me ideas as to what I can use as a deterrent, to make the leaves taste nasty for a while, or  . . . . . . . . . . . . ?
Several gardeners round here have mentioned using black soap.  Does anyone have any experience of that?

[peanuts]

[strawberry1]

diatomaceous earth has worked well for me