Accidentally sourced .bash_history

If it's a local machine, start a new terminal and kill the shell in question.

If it's a remote machine, ssh in and kill the shell in question.

You can hit Ctrl+Z or start a subshell using :!bash, and then kill the shell. Specific steps:

Ctrl+Z
kill -9 $$

:suspend (or acceptable abbreviations thereof) may function equivalently to Ctrl+Z.