We are currently providing a captive portal option “Allow to Skip Sign-in”:
When it is checked, a “No thanks” link will be displayed at the bottom of the social sign-in page. Clicking it will sign in the service without needing to sign in any social network. No personal data will be collected in this case.
If you do not have the social mode but other modes enabled (e.g. e-mail), you could enable the Open Access at the same time. So if a guest does not want to provide his/her e-mail address, he/she could choose the Open Access mode.
For allowing guests to deleting their data, we have the following proposal. After a guest signed in any one of the modes (except the Open Access mode), the page will display a link “Remove my personal data” at the page’s bottom:
Clicking it will remove all the information of the guest that the system collected. What do you think?