You actually have several options here. You could provide the guest with a watered-down version of their drink. You could also provide the drink while making arrangements to get them to their house. Another choice is to refuse the request by saying you have run out of alcohol or by explaining they have had enough.
Although reasoning with a drunk guest seldom helps, it is worth a try. You could also start providing them with additional food, coffee, or water to help their body absorb the alcohol. Finally, you will probably want to ask someone else to assist you, as it does help to have another person involved.