I was thinking the other day about a service called Verifier that could be implemented with something like this.

Verifier would allow a person to take a token of some sort from a website, and send some eth to the Verifier. Now the service knows that the person has put in some stake, and can grant them an account (this is as a sort of abuse protection).

If the person wants they can take their money back, but the website can find this out and cancel the person's account.