I'm dying to meet a BNP canvasser; I have a couple of terriers, one a feisty Jack Russell with finely developed political instincts...
Seriously, the correct tactic is to invite them in for a cup of tea and a chat, feigning interest in their party. Keep them there for a couple of hours if possible, discussing all aspects of their policies in great detail. Think of how many other doors they won't have time to knock on.
Tim