I guess these guys are sending a message "asking" about lessons, but they have no intention of paying for your time, let alone learn. For any inquiry, I reply with something like, "Feel free to check my schedule and see if any times are suitable for you." Usually, if that person is sincere about lessons, they'll book a session.
I know this doesn't deal directly with the flirting problem, but that usually weeds out the general time-wasters. If they message you again, just ignore them. There's no reason to be polite to people who deliberately ignore what you've written.
If you really want to write a reply, just "Thank you for the message, but I'm not interested" is enough. A more direct message would be, "I'm sorry, but I do not believe you're sincere about learning Chinese."
If they are persistent or rude, block them and definitely report them. Think of who else they might be harassing.