Quote Originally Posted by kalinka_vinnie
prevented or kept would mean that he couldn't participate at all
hampered means that he could participate, but it made it more difficult.

That nuance I think is important, or maybe помешала means the former?
In this context, that would be my interpretation. That is, he was not able to participate at all. as opposed to, say, "мешала" which would mean that it was harder but he did work nevertheless. IMO.