Connect to OLT and go into the configuration mode:
enable config
Let’s look at the current configuration and find the MAC address of the ONT you want to delete:
show running-config
Choose the interface (port) on which ONT is registered:
interface EPON0/1
And delete ONT, for example with mac address 8014.a8b4.eb2c:
no epon bind-onu mac e067.b336.88e8
Done.