You have been very lucky then as I don't think I have ever been asked only once for anything on DCL. On the first day everyone we encounter wants to sell us some type of package (water/wine), the CM's are all over the top deck asking. Also, I normally get the wine package question for the first two nights at dinner, the second night is usually a 'are you sure?' question. As I mentioned earlier they are often set up at each port selling water.While I have been asked if I want to purchase a wine package ONCE per cruise on DCL and there are ladies from the spa out, the major difference I have found with DCL is that once you say no if asked they leave you alone. And they are not jumping in your face with things like they were on RCCL. Literally last summer I had to ask three crew members on RCCL to please move so I could enter the buffet because they were completely blocking my way waving their whatever they were selling in my face.
Your right, the spa ladies tend to hang out at the doors to the adult only area and hock their wares. Can't say they only ask once there either because when they are out, each time I leave and come back they ask again. Everyone is friendly but they are there.
So far I have only sailed on the Oasis once however I have not had any issues with pushy sales people. I just say no and move on, they don't follow me. Maybe I will see more when we go back. I'll still just say no and walk away.