When you go in for your facial or some body treatments, you'll likely encounter some form of product pitch, so be prepared. If the aesthetician starts mentioning products during your service, just say, "I'm happy to hear your recommendations, but I'm really here to relax, so please save them for the end."
After your service, if you don't want to purchase anything, ask the aesthetician to write down the products she's recommended and let her know you'll think it over. Once you say you're not interested right now, she shouldn't say another word. If they push it, you should consider telling the spa manager.