If someone has a knife, believe it or not, the best thing is to beat them up aggressively. Maybe you get stabbed but probably not, if you are a decent boxer with power in your hands. The sooner you strike, the better. Getting stuck once is bad, but you stand to get stuck a whole lot of times, otherwise. If you can land one punch and keep landing them, you can prevail. You don't want to get in a situation where you are facing off someone with an already drawn and ready knife, unless you are much taller and stronger, then you better get out of there. If you can't, pick up a chair, a bar stool, something longer than the blade and jab them in the face with it. Don't swing it, because they can time the swing. How effective you will be depends on the power in your hands. Strikes need to do physical damage, broken nose, orbital, knockout, etc.