I'm not sure why you would refund something the guest agreed to when they booked. Now the guest will do the same thing to another host and get mad when that host does not agree. However, go the the guest's message thread. Look in the left column. About 1/2 way down is a link, "Send or request money." Click that and follow the prompts. My suggestion though, is to not do it until after you see how the guest leaves your home.