I'm afraid that over the years when Busking I've been on the end of verbal abuse and threats from Big Issue sellers.
I think they are a relatively small percentage of the hole and I generally think the organisation is a good idea.
The thing to do when this happens is to clock the Guy's Identity number go to the local Big Issue office and report the incident. In my experience they take this sort of thing very seriously and will take action against the individual concerned.

PP