Closing a stale SSH connection（关闭过时的 SSH 连接）中介绍了利用 SSH 转义序列来关闭失去响应的 SSH 连接。

也就是说在 SSH 终端输入 ~. 会直接中断 SSH 连接。
经过试验，确实有效（使用 SSH 代理建立的连接就没效）。

所有 SSH 转义序列：

[root@dell ~]# ~?
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
(Note that escapes are only recognized immediately after newline.)