One possible option, assuming one is using YazFi, is to try blocking the client's internet access using custom firewall rules. The YazFi GitHub (
https://github.com/jackyaz/YazFi) has a section covering Custom Firewall Rules with a few examples that deal with accessing main LAN clients. One can try to use IPTables scripting to block internet access by MAC address using that YazFi custom firewall rules steps. I did a quick test and it seems to work. No idea if it impacts other elements of YazFi or causes other issues with the router firewall/IPTable rules. No idea if this is even the correct coding to use, but it does seem to work for me in my quick test. The YazFi client that is being MAC internet blocked can connect to the YazFi guest network (my example blocks them on Guest #1 both 2.4Ghz and 5Ghz)
BUT they have no internet access. Remember to restart/reapply YazFi settings after setting up the file containing the blocking code.
As always one proceeds at their own risk!!!
Code:
#!/bin/sh
iptables -I YazFiFORWARD -i wl0.1 -m mac --mac-source A0:C7:AA:C8:BF:B7 -j DROP
iptables -I YazFiFORWARD -i wl1.1 -m mac --mac-source A0:C7:AA:C8:BF:B7 -j DROP
One should note however that spoofing a MAC address is possible if one is so inclined. So it will be possible to bypass such MAC blocking if one was enterprising enough.