I'm not altogether sure why I should feel pressured to stop walking and tell Big Issue sellers (politely but informally, of course) that I'm not interested in their magazine, thanks.
I don't call into WH Smith every time I pass just to tell them I don't want any of their magazines.
Ok, sure, maybe they've had a rough time of it and deserve some courtesy. They're doing a job that no-one would never really actively choose to do, in an inhospitable environment where they're subject to constant abuse.
But then again, so are Big Issue sellers.