First of all, welcome to the Negril Message Board Simon! It is great to have you join us and you will find helpful information here to make your trip the best possible! And sorry about your thread being highjacked a bit...
In regards to your interactions with the local vendors or higglers as they are called, you will have no issue at all with any of them understanding "no thank you", "no thanks" or a simple "no". All are equally fine to use and none will be confusing for them to hear.
To Craig and Northcoast/Jajewel - Jamaicans are taught manners in school and at home. While the use of the "No thank you" is not used in everyday speech in the interior of the country, in a more populated place, be it a tourist area or a city like Montego Bay, Mandeville or Kingston, it is not uncommon to hear "no thank you" in everyday speech. And to those from the interior who are trying to earn a trade in the tourist areas, any new vendor will pick up the usage of "no thank you" in a matter of days. To suggest that Jamaicans, especially Jamaican vendors in a tourist area will be confused by such usage is insulting. I know Jamaicans who were insulted by the comments on TripAdvisor.
With the internet and cable tv being more readily available, showing the interactions from people from all over the planet, the case of Jamaicans even in the interior of the country not understanding what something like "no thank you" means is becoming a thing of the past.
And I speak about this from a perspective of nearly 20 years living daily in Jamaica, and observing how people interact from all corners of the island.