Free offer signup question

I am working on a website. The website will be giving the users information. I want to be able to give the users a free trial of about 0-100 free xxx.

How can i make sure that the users dont make mupltiple accounts, in order to get more then 1 free trial?

It's a hard task really. There is no secure way to block such possibility. You can use IP blocking/email verification/making service in a way that you have to have just one account.

Thanks,