tl;dr: Press Enter, then ~, then ..
Sometimes, ssh will hang with the remote not responding to control characters.
Ssh actually supports escape characters to send functions directly to the ssh
client, a list of which are available by running man ssh:
ESCAPE CHARACTERS
When a pseudo-terminal has been requested, ssh supports a number of
functions through the use of an escape character.
A single tilde character can be sent as ~~ or by following the tilde by a
character other than those described below. The escape character must
always follow a newline to be interpreted as special. The escape character
can be changed in configuration files using the EscapeChar configuration
directive or on the command line by the -e option.
The supported escapes (assuming the default ‘~’) are:
~. Disconnect.
~^Z Background ssh.
~# List forwarded connections.
...and so on.
As it states, the escape sequence is a newline (pressing Enter), followed by the escape character (defaults to ~), followed by your command. . will disconnect the ssh session, Ctrl + Z will force the session to the background, and so on.