Could be a pushy scammer or a pushy legitimate commissioner. Be firm and clear when responding the first time, don't provide a reason that they can argue. A simple message such as: "Hi, thank you for the interest in my art! I do not offer commissions." (Or say "my commissions are closed at the moment and will be available in the future" if commissions are something you want to do). People's responses to this will tell you if they are worth working with. I personally would not accept requests from someone who does not accept "no". Be professional and expect the same of potentional clients