When I get approached via email to complete a survey I always respond by saying that I would be happy to participate and then end my response with when can I expect to be paid.
That usually is quite successful at stopping further emails from that company.
Oh and forgot to mention, HP actually sent me a CAD$50 e-voucher that I can use on their online store after I completed a quite lengthy survey - mind you after I read the fine terms it did mention that they would be sending the e-vouchers as a thank you to all who completed the survey - but it goes to show you some companies will pay for the information.
If everyone asked to be paid then the research industry would be forced into paying people for their information, afterall they make millions.
FTL