I just check absolutely nothing on the questionnaire (because it's all just a way to sell you things), and then if/when they start showing products at the end, just smile and say politely, "I'm really not interested. Thanks!"
That's usually enough, but once I had to repeat myself, with a slightly concerned expression on my face, like I wasn't sure they were listening to me. That was the end. If they continue after being told you aren't interested, that would be grounds for a complaint, but it's never gotten that far (for me, anyway).
People hate feeling like they're being rude, so guests don't like to say, "no." Sales people try to use that to sell you things. It's helpful to use a polite phrase that means no, but isn't rude, such as "I'm not interested. Thanks." If the spa employee continues, now they're being rude. You don't owe them an explanation. Just look mildly surprised and say, "I'm not interested" again. If they try again, ignore any questions relating to products and ask if you're done, or ask for directions to the changing rooms.
If they should start rattling on without giving you a moment to speak, just raise your finger like you have something to say; they'll usually stop shortly. If they don't, again, now they're being rude, and you can interrupt them to say, "Excuse me, I'm really not interested, and I have an appointment to get to. Could we wrap this up? Thanks."
Keep in mind that the folks who work in these spas are trained to offer products, and often get performance reviews based on how well they sell them. It's not their fault; they're just trying to keep their job. Don't get mad at them, get mad at Steiner (the company that runs all the cruise spas). Or better yet, don't get mad at all, just prepare yourself. Use these techniques and don't waste their time or yours.