How will you handle the situation when the intoxicated guest asked you to order a case of beer or bottle of hard drink?

Since the question specifies "purchase more" we must assume that you are referring to a client or customer of a business establishment and not to a true 'guest' (as in, someone visiting your home).

Unless you can cut them off by refusing to serve them, or 'sweet-talk' them into leaving, your only other option is to call law enforcement and have them escorted from the premises. If you are found to be knowingly serving intoxicated patrons, your ABC license could be in jeapordy. Even worse, if the intoxicated patron causes injury or death to someone after leaving your establishment you could also be sued for civil damages by the victim(s) under "dram shop" laws.