Dealing with pushy charity people

I have been approached by a common charity-like organisation by Farringdon station. I am getting used to this happening every time I go through a tube station but this time, after I said “no thanks” the guy wouldn’t leave me alone and would basically start chasing me all the way into the entrance. I started to get very upset considering that I continued to say no and the situation started to get overwhelming. I feel uncomfortable being approached every time I am travelling, which is one thing, but to ask these people to leave me alone and them continuing to chase after is getting a bit too much. I started to raise my voice and avoided eye contact but it did not seem to matter. To make my point here, all I’m saying is that it seems like basic boundaries such as saying no and walking away does not seem to be enough anymore, not in my case at least. Any tips or similar stories on how to handle these situations?