How do I get this guy to stop messaging me on Facebook?

It's a guy I used to know from high school. He needed permission to join a group that I happened to be part of so I approved him, and he used that as an excuse to message me. This started Saturday afternoon, and it's still going on (although I have started lagging in my replies). I would have used some excuse at the end of Saturday, but he hadn't replied to my message after a few hours, so I assumed he was done, until BAM he messaged me right before I went to bed, so I ignored him for the night cause it was late. One time I went to a convention with my friends freshman year of college and he happened to be there so we chatted a bit, afterwards my friends said they could tell he was interested in me. Well another time he also asked me if I wanted to do lunch together and I said no. So I'm afraid he's going to ask me out eventually, but nothing yet, and it's annoying. He keeps asking me about my life and I answer - but I don't ask him anything and he still manages to come up with a reply. I really hate messaging in general, and I really feel like he's going to ask me out. What do I say to him to get him to stop? Sorry for the lengthiness.