Student books 5 classes per week, attends one and postpones the others because they don't want to "lose hours"...how do you deal with them?

I have this student who keeps booking slots without logic, once they booked 5 slots in a day, just to cancel them the day before, then they postponed them all to the next week, one slot per day (which they will never attend)...how do i deal with such behaivour? I explained them the situation and explained quite explicitly that they should only book classes that they want to attend. If they dont listen, is a block the only solution?