Usually just say no thanks and hang up. I'm never rude, as these poor people on the phone are just trying to make money, and usually don't have any other option. But I'll NEVER buy anything. I don't want to encourage them. I'm also agressive with their employers to get them shut down, especially as I'm on the no call list, and shouldn't be getting calls. With companies I already do business, I've gone as far as cancelling my accounts, or threatening if I ever get another call, but you have to talk to someone more senior.
1999 e46 328i Ti Silver / Black[retired]
2007 e90 335xi Jet Black / Black[retired]
2011 e70 X5 35d Vermillion Red / Cinnamon
2011 e92 M3 LeMans / Fox Red extended