You can buy their products online
here, I wonder if they charge more on the ship?
I have found the best way to deal with Sales Pitches is to just say no. Don't give them any excuse, just say no repeatedly and leave it at that. As soon as you break out some excuse ("well, I don't now if I'd really use it" or whatever), they'll have an answer for it. They've heard them all and know a response to all of them. That's how they break you, they get you to the point where you have no more excuses and then you feel like you should buy.
Instead, just say no and leave it at that. If they ask why, just smile and say you just don't want to. Don't give them any ammunition to work with. They'll get frustrated and give up pretty quickly.
If you think you may want some of their products, go ahead and say no anyway. Go in planning on saying no to anything. If something truly sounds good to you, when you get home you can look into it and order online, or better yet find try some cheaper products and see if you like them as well.
And most importantly, if you feel the person was too pushy and was trying to sell you something rather than just presenting some products you might like, write disney a letter after the fact and let them know. I don't mind being offered some products, but I would mind a hard sell and I don't think it has any place on a cruise ship. If Disney get's enough complaints, they can probably put some pressure on Steiner to knock it off. Or, as other cruise lines are starting to do, they may consider brining the spa services in house.