Buyer wont stop messaging me

She had bothered me over months about creating an art piece of someone, and when I happened to finally create the art piece for sale, she wouldn't stop sending me messages. She purchased from me (which im so grateful for) however, every single message she sends I have to reply to. If I don't, she sends the message again like i didn't read it the first time. And now she's asking me stuff unrelated to the order. She sounds like she's 13 though I had a friend tell me she's well known in the fan group I created the piece of. Is there any way I can nicely tell her to stop messaging me or something? I hate to block her or anything since I have an open order with her currently.