I had one years ago who used to do this and he was a good regular, but it drove me nuts and it's so intrusive - I got to dreading looking at my phone when it went because there would just be more of the same crap (it was before the days of smartphones and blocking texts) and obviously you can't switch your phone off because work.
I used to occasionally answer asking when it was he wanted to book and for how long, but in the end I told him that I don't offer a text chat service and suggested he look online and find somebody who did if that was what he wanted. It also sometimes helped to remind him that I was working - leave a bit of a delay and then reply with something about how you have a really busy day and another punter due soon, so you just need to know when he wants to book? He won't, and IME it keeps them quiet for a bit.