Need help with the Create User Wizard control and restricted visibilty

I'm creating a "Wizard like" user interface that uses the Create User Wizard Control.

 Basically, after client successfully makes a purchase they are forwarded to a an .ASPX page which contains the Create User Wizard control.

After they create their account they move on to other registration forms. My question is: How do I restrict them from going back to that page (bookmarking the page or directly hitting the URL) and creating additional accounts? I have a few ideas, but I'm not sure they are the best way to go. Can anyone give me some advice / ideas?

 Thanks, Robert
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