Wait until they get to the top of your bamboo wigwam (or whatever you are growing them up) and then just nip off the bud at the end of the stem with your fingers. This will stop the 'vine' growing up, so it can put all its energy into making big beans.
Thanks sddsddean.
Will try this. Thought you had to do something like this but wasn't sure. Hopefully we will get a good crop. Supposedly the side shoots will grow further (with more beans)