Terminating telnet is quite easy. Just follow, what telnet itself writes on screen:
Escape character is '^]'.
But, hey, what does that mean? This is the defined ASCII character for "Escape Sequence", which is in value 0x1B or 27 in decimal. You might think that terminating telnet would be possible by just using the "ESC" key on your keyboard, which is exactly the keyvalue. But this key-press will not be recognised by the telnet session.
The solution is to generate the escape-sequence with the following keys ("+" means using the keys simultaneously):
STRG + ALT GR + ]
Note: This is how it works on a german keyboard. It might be different (in case of ALT GR) on other language maps.