You need to segregate those clients on a separate LAN (or VLAN). That means:
A second WiFi Access Point, or a WAP that understands VLANs and lets you assign the different SSIDs to different VLANs
A switch that understands VLANs, or a separate interface on the pfSense firewall box
I'm doing a mix of the two methods, I have VLANs on my network switches, and one of those VLANs is for "guests". The guest VLAN connects to a cheap consumer WAP which doesn't understand VLANs, but the network switch handles that issue.
Now that the guest WAP is on a separate VLAN, I can assign limiters in the pfSense firewall rules to any traffic coming from the guest VLAN and going to/from the internet.