If I haven't heard from the person before then I will respond nicely to "hi" - as in replying Hi, would you like to make a booking today? If so could you please give further details? Many thanks Hadley.
If they just reply with "hi" again or if from my previous messages I can see they have done nothing for the last 4 months but send the word hi, then I just text anything they send on repeat back to them. As in:
Them: Hi
Me: Hi
Them: Hi
Me: Hi
Them ?
Me: ??
Them:
?
Me:
Them: bby?
Me: bb ?
Not a very stimulating game admittedly but mildly amusing when you're bored. And hopefully gives some idea back to them of how mind numbingly soul destroying it is to try to have a conversation in this vein.