Are those two IP addresses connected to DIFFERENT APs? If yes, write firewall filters in the 'forward' chain to drop traffic between customer IP addresses after permitting traffic between customer IPs and their gateway. If no, you've been making mistakes implementing what you've been told in this thread, and should post your configuration.
"It doesn't work" is NOT a sufficient answer. Show WHAT doesn't work, how you're testing, what you're expecting, and what you're getting instead. People can't just randomly guess at what you're doing, not giving enough details wastes everyone's time, including yours.
Specific answers require specific questions. When in doubt, post the output of "/ip address print detail", "/ip route print detail", "/interface print detail", "/ip firewall export", and an accurate network diagram.