I've always pinched my tops off when they are nicely flowering and set their first few beans.
Does not seem to stop any growth, as always get a bumper crop.
Never really had any problems with blackfly on mine over decades of growing in same or other parts of the garden plots.
Did mine just over a week back and this weekend will picking my first crop.
Planted my Super Aquadulce and Bunyards Exhibition out mid March