I would just take the bull by the horns and tell him plainly “no”. It’s your endeavor, your project and perhaps he can do his own comic book design if he feels like contributing more. Perhaps he’s not getting the message because your answers are vaguely negative and not to the point. Or, he is just deciding to ignore and persist, hoping you’ll give in. But since this is your project, you have to draw the line (literally) and tell him where he comes in. This will be good practice for you to say no to people and be firm for the future when you’re running your own business. :)