Move to the home directory
$ cd ~

Move the .zsh_history file into another .zsh_history_bad file

$ mv .zsh_history .zsh_history_bad

Write all printable strings into a new .zsh_history file

$ strings .zsh_history_bad > .zsh_history

Finally, reload the history.

$ fc -R .zsh_history

Happy ZSH-ing!