I do not think that it will be easy to get rid of the BASSA militants. If they work outside strike periods and behave well, then BA can do nothing legally to dismiss them. And frankly, this is quite proper.
Nor will they leave BA no matter how much they hate management or the company as they won't get a better job outside.
So, what is Willie Walsh doing about them? He is making them less important. By external recruitment (both permanent and temporary), by internal recruitment and by training VCC he is - admittedly at some cost - adding numbers to cabin crew. So, if they strike again, the impact will less severe than any of the previous five strikes. There will come a point when even BASSA will know that they have lost and will never be able to hold a gun to BA's head.