Issue with Rescheduling Guest

[Anand S.]

Recently a guest wanted to cancel their reservation but would've only received a 50% refund due to my cancellation policy. I offered to reschedule them as an alternative. They decided to reschedule to a date over 2 months out, which I agreed to. I updated the reservation and they accepted. Once accepted they were able to cancel and get back all of their money since the new dates were outside of the penalty window. 

I now have another guest asking to do the same thing but I do not want to get outwitted again. How do you folks deal with this? This is on VRBO.