Captive portal with shared laptop

Hi Everyone, I have FortiWiFi 30E for offering a free wifi access. Visitors simply reach the captive portal and provide their email address (email collection).

All run well but as we also provide a shared laptop (chromebook) only the first visitor actually need to give his email !!! Other visitors never see the captive portal because the laptop is already known and connected. This happens even if each visitor use a new guest session on the chromebook.

So the question is how to identify the user, not the device ? So that all visitors will have to "authenticate" on the captive portal even through the same device ? Any thoughts would be helpful Regards!