One Poll Cash Back is registered in the UK and so its marketing practices should comply with the Privacy and Electronic Communications Regulations 2003. The Information Commissioner's guidance states (point 119):

Organisations must not disguise or conceal their identity in any marketing texts or emails, and must provide a valid contact address for individuals to opt out or unsubscribe (which would mean consent was withdrawn). It is good practice to allow individuals to reply directly to the message and opt out that way, to provide a clear and operational unsubscribe link in emails or at least to provide a freephone number.

Although I haven't seen any of the e-mails, what you're saying suggests that OnePoll Cashback is in breach of the regulations. You should consider lodging a complaint with the Information Commissioner's Office. For more information you can also phone them on 0303 123 1113.

In your complaint you may want mention that One Poll Cash Back website requires people to enter their name and e-mail address just to find out how the website works (and presumably have their details added to a junk mail list). This is a breach of the Data Protection Act 1998 (which is also enforced by the Information Commissioner's Office). The Act states that organisations should process personal details fairly and lawfully, for limited purposes, and that the information collected should not be excessive in relation to the purpose for which they are processed. OnePoll Cashback appears to be breaching all these principles. It's hard to believe the company is actually registered with the Information Commissioner's Office (its reference number is Z7981900).

Also worth mentioning is that One Poll Cash Back has carefully hidden its terms and conditions. You can find the terms via Google but from the website's home page there's no link to information about how the website works. Interestingly, the terms and conditions do mention opting out; point 10 (of 51!) reads:

[hellip;] Your email address will be used to send you regular Rewards Program member's [sic] offers. You will be given the option to opt-out [sic] of receiving these members newsletters.

Finally, there are two lessons to be learned here. The first is to never ever sign up to websites like these. If you can't find a website's privacy policy, or if you don't quite understand what you're signing up to, you really should not be handing over your personal details to greedy marketeers. Instead of trying to save a couple of pennies on buying things from chain stores you'd be better off buying locally / organic / fair trade.

Secondly, if you do need to give your e-mail address to a potential spammer you should use a separate e-mail account. E-mail accounts are free and easy enough to set up; keep one for relatives and friends, and another to sign up to things. That way you keep your private e-mail account private, and at any point you can delete the spam account (once you find the spam starts taking on monstrous proportions).