Is that using InControl? If so, you can force the client devices to disconnect from the WiFi every x minutes (say 30 minutes), but set the Reset after session ends to zero minutes - this would allow them to log back in again. You could also set the DHCP lease time to 30 minutes.
If someone did configure their device with a static IP address, after 30 minutes, the Captive Portal would automatically log them off. They would be able to log back in again, but if there are others wanting to connect at the same time, they could well be given the IP address of the device that has just been logged out. Note - I believe this is new with InControl 2.8.4 so if you setup the Captive Portal a few weeks ago, this option probably didn’t exist.
I hope this helps,