user contributions, purchased databases, paid moderation and web crawlers.

I happened to have a can of soft drink in my had so I punched the UPC in and it was not found (unsurprisingly, it's a small northern soft drink company in the UK) so I hit Add Product and then it asked for a signup.

I think you'd get better buy in if you could add the product first then sign-up or not have to signup at all but put those entries into some kind of moderation system.

wow, thanks a lot for your thorough advice :) tbh I really hate putting road blocks in front of people who would like to contribute to the database, however we were the victim of a huge spam attack with completely random ip rotations. I will eventually remove the mandatory sign up once we have a strong moderation system in place, this shouldn't be too far from now. Thanks again, I really appreciate your comment :)

if people are rotating IPs to dodge your blockers, maybe you could shadow-ban them instead? that way they don't know that the IP is bad.

That's true; the mandatory sing-up is a temporary measure, I'm sure there are a lot of ways to fight spammers including the method you mentioned. Thank you for your suggestion.