How can I disable telnet on BT Home Hubs?
You don't need to.
Port 23 is blocked on all BT home hubs.
Source BTCare Community Forums ... Port 23 for telnet
Historical note:
Telnet shell access was available in versions (up to 6.2.2.6) on the BT Home Hub 1.0 with appropriate user permissions.
BT subsequently locked down telnet for "security reasons".