The height of broad beans depends on variety and soil fertility. There is a natural limit to the height of the broad bean stalk.
You do not have to take the top out of the stalk. One reason for doing it is to stop them growing so tall that they will fall over on very fertile soil. The main advantage of taking the top out is that the risk of damage by blackfly is reduced.
We take the tops out when we see the first black aphid on the growing centre of the bean. This is easy to see as they usually concentrate on one plant and form a shiny, black mass.

Thanks for your question

TopVeg

Thanks for the video about taking out the growing tops of beans! I wondered how high broad beans should grow before you do this? Mine have grown lots of flowers and the beans are poking through. Do they just get bigger and bigger and produce more beans if you leave them?

Thanks!