Normal keys are forwarded over the ssh session, so none of those will work. Instead, use the escape sequences. To kill the current session hit subsequently Enter ↵, ~, ..
(Have in mind that in international keyboards were ~ is set to be a composing character you have to hit it twice: Enter ↵, ~, ~, .
More of these escape sequences can be listed with Enter ↵, ~, ?:
Supported escape sequences:
~. - terminate connection (and any multiplexed sessions)
~B - send a BREAK to the remote system
~C - open a command line
~R - request rekey
~V/v - decrease/increase verbosity (LogLevel)
~^Z - suspend ssh
~# - list forwarded connections
~& - background ssh (when waiting for connections to terminate)
~? - this message
~~ - send the escape character by typing it twice
In the command line you can get help how to add or remove port forwards typing -h:
ssh> -h
Commands:
-L[bind_address:]port:host:hostport Request local forward
-R[bind_address:]port:host:hostport Request remote forward
-D[bind_address:]port Request dynamic forward
-KL[bind_address:]port Cancel local forward
-KR[bind_address:]port Cancel remote forward
-KD[bind_address:]port Cancel dynamic forward
(Note that escapes are only recognized immediately after newline.)
You can close the list of Escape sequences by hitting enter.
Notice that because hitting ~~ causes ssh to send the ~ instead of intercepting it, you can address N nested ssh connections by hitting ~ N times. (This only applies to ~s that directly follow an enter.) That is to say that enter ~ ~ ~ ~ ~. terminates an ssh session 5 layers deep and keeps the other 4 intact.
source: https://askubuntu.com/questions/29942/how-can-i-break-out-of-ssh-when-it-locks