How to detect multiple registration to the web from a single user

use php and mysql, My situation: users register to the web by开发者_开发技巧 entering username, email, password , birthday, sex and then activate account by clicking the link in their mails. Email and password are used in the log-in process.In database,I also record IP address and date-time of registration. The problem is some users registered too much accounts to do something on the web as I limited one account per one click. The easy way is to check their IP address is existed or not, but in some academy organisation or company use the same IP address for all computers. How to limit their registration and not to effect everyone who are those in the same ip address?

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Think of a way to block automatic registration. That's usually why you see the many registrations pattern. Robots crawl and register any site they find and can figure out.

I usually encode the FORM and use JS document.write() to print the form. Or use a hidden field filled by JS using and external obfuscated file. Or a CAPTCHA...

See if this does not fix your problem...