I think you have to let four trusses form before you pinch out the tops, feed with tomato feed when the little tomatoes form. I am having the same trouble with broad beans in my allotment, but as they were a freebe I'm not too worried about pulling them up. I'm no professional when it comes to food plants, but I give it a try.

Hi, Cammomile @ Gwynjim, had the same problem with my broad beans, Donot pull them up, just spray them with a soapy solution, it worked for me, have had quite a good crop this year, considering the small size of my veggie plot, good luck,....

Hi, you should have kept an eye on the broad beans as soon as you see blackfly pinch out the tops and bin them that usually does the trick, found a better spray than soapy water, make a brew in a bucket with mint leaves and boiling water let it cool strain into a sprayer bottle works much better.good luck,oh by the way if anyone nearby is growing broad beans with blackfly drop the hint for them to pinch the tops out or they will spread to yours.