Empathy chat client stores chat history in your home directory under
You will see a directory for each account you have configured and inside each are directories for each person you have spoken to.
To delete the chat history completely simply delete the contents of ~/.local/share/TpLogger/logs. Or you can be more selective and choose which conversations you wish to remove.
To prevent Empathy from recording chat history in future, In Empathy, select Edit > Preferences from the menu. On the General Tab, untick the option "Log conversations".