Fiery Greatsword cancel auto attack animation
If you mean stop it mid attack, the default key should be Esc. This cancels any move with a cast time. You can remap it.
If you mean stop it from doing it’s auto-attack automatically after you press it, you want to hold shift (left) and right click the skill with your mouse.
Now it’ll only attack if you manually press it. Useful for saving charges. Do the same thing to turn it back on.