When I worked in Glasgow, the ones in Buchanan Street were a right pain. The moved every other day so you weren't always sure where they were, and they didn't always wear brightly coloured tops. Noticably, it seemed to be the same people working for a different cause each week. I got pretty good at avoiding them, usually making sure I walked behind someone else so while the Canvasser was distracted trying to talk to them, I could walk right past. Failing that, I would tell them I had spoken to one of their colleagues down the Street and had signed up with them. Usually they would thank me for my support and leave me alone, though once one of them challenged me to tell them which organisation they were working for that day. Thankfully I had been lucky enough to have read the logo on her jacket 10 seconds before she covered it up.