Leo,
If you have a A400/A800 or D800/D1600/D2400/D3000, then there is no way to disable the telnet function.
If you have a 2nd generation tenor (as/ax/dx0 then you can disable telnet access by going to the ethernetinterface (ei) and setting the management access to 0. Once this is done and you have submitted the change, you will no longer be able to access the Tenor from telnet, only from either config manager web/access or from the console port direct connect.
Another way to limit this is to block the ports in your firewall/router.