I have found the following to be effective on my broad beans.Grub out
the blackfly on the affected plants, then go round the clean plants and
rub your grubby fingers gently on the stems.It seems to send a message
to the ants not to farm the aphids there. If you are squeamish wear a
pair of Marigold plastic gloves. This method is organic,it is free, and
it is what our forebears did.