Peplink Balance 1350 Captive portal Multiple devices?

Good day ,

On this site we are using the captive portal of the peplink and we are giving them guest-Accounts of like a day or a hour.

So far so good but now the problem occurs that they are selling these guest accounts we provide for them and that you can logg in on multiple devices with one guest account.

Model Peplink Balance 1350
Hardware Revision 1
Serial Number 182C-113C-7DAF
Firmware 5.4.9 build 2573

I wonder or there is any solution for this ? Any setting I missed "? Any new firmware that has this option ?.

Thanks in advance.
Greetings ,