The most gracious way for a hotel employee to refuse a gift of alcohol from a guest is to cite hotel policy. For this reason, it is important to have a hotel policy regarding what sorts of things employees can accept from hotel guests in the way of tips or gratuities.
A hotel certainly does not want its employees to be accepting things like drinks bought for them by appreciative guests. It does not want to have employees drinking on hotel premises, regardless of whether a guest has invited them to do so. Therefore, it should have a strict policy against employee drinking.
An employee could then use the policy as a way to graciously decline the offer of a drink. The employee could say something along the lines of “Thanks so much for the offer. I’m glad you liked the service we provide. However, I’m really not allowed to accept any drinks here in the hotel. Thanks again.” By saying something like this, the employee can avoid angering the guest.