From a slightly different angle, but still on the subject of saying no to Thais:
The father of our landlord asked us if we could teach his grandson English on Saturdays or Sundays. 'No', we politely said in Thai, and explained that although we weren't at work, we still had a lot of correcting to do at the weekends. He persisted saying that he saw us at the house, so we obviously had free time, so his grandson could just be with us and learn English. 'No', very politely again, we really don't have the time or inclination. He still persisted, and it wasn't until the third 'No' that he finally got the message. He wasn't annoyed or upset, he just said OK and got into his ratty pickup.
I had a chat with my Thai colleagues about this and they chuckled and said, 'That's the way we do it in Thailand'. If someone asks you to do something you don't say 'Yes' straight away. You say 'No', even if you mean 'Yes'.

That way you make the person asking the favour feel that you're
really doing them a big favour. After the third 'No' they know you really mean it.
We've found out that the 'three-time rule' also works when we ask for something to be done at work. Blimey, it's a bit confusing here sometimes...
VS