Here in Spain there are a couple ways to say no. Typically you combine two of these. If it is a really serious NO you may use more.
1. No, no, no. This is said firmly, three times.
2. Finger shake. This comes with a remonstrating look. This one makes me laugh when it comes from my six year olds.
3. Clucking sound. This is kind of like the sound you make when you want a horse to go faster but it made with the front of your mouth not the back.
I have discovered that “no thank you” is taken to mean “I’m too polite to say yes”. Your “no” will be accepted if it is just no.
As I am learning to say no in Spain, I’m also reflecting on how I say no in the USA. I’m thinking I might need to say no to some things once I return.