Saying "No gracias" is usually enough. If that dosen't work, ignore them. If that dosen't work then go nuts on them. I'll share two examples of what happened to me in Sosua last year.
A friend and I would walk up and down the beach pretty much on a daily basis. We would pass the shops but weren't really interested in buying anything. Of course the vendors try to get you to come in, but we'd smile and continue on. This one vendor kept wanting to sell me sandals all the time. "No gracias", smile and on I'd go. On the third day of this I was getting a little annoyed. Once again this guy is like "come on mami, look at the nice sandals" I had had enough, know if you saw me walking down the street you would assume that I was a gringa, white, blond, speaking english. So, I turn to this guy and yell "NO QUIERO LAS MALDITAS CHANQUELTAS!!!! DEJAME TRANQUILA PESAO E MIERDA" Yup I turned Cuban in an instant. Needless to say that the next time I walked by his stall he didn't try to sell me anything. :classic:
The second one was with the women that do the braids. I have long staright hair, and don't like braids. I don't think that it looks good on people with my type of hair, especially when they are on vaction. Blond, thin haired people with sun burned scalps and faces, is just not sexy in my book. Not my thing. Just my opinion. So, here comes this woman with a book showing the braids, I say "very nice but no thanks". Keep on going further down the beach, the same woman, once again, "No gracias". She must have come at me with that book about four times while I was on the beach. Each time I was pleasant and said no thank you. I imagined that I would not be propisitioned again. WRONG!!! We were standing in front of the beauty parlor across from La Roca when here she comes again with the stupid book. I totally flipped and went Cuban all over again. " VEN ACA CHICA QUE CARAJO NO ENTIENDES DE "NO QUIERO LAS TRENSAS" ACASO ME VES CON NESECIDAD DE TRENSAS. QUAL ES EL LIO. YA TE DIJE QUE NO LAS QUIERO VETE A JODER A OTRA TURISTA. MIERDA." Then I kept on ranting in english. My friend was like "OK Minni I think you got the point across, you can calm down now." Not one of my better moments in Sosua but effective.
So, yes be nice and pleasant, say no thank you and keep going, ignore them if you think it'll work, but be prepared to be rude if you have to. These were the only two things that ticked me off. Most of the vendors were ok, when they got a no. But there will always be the ones that think that they can get you to buy. Just try to have a good time and don't be coerced into buying something just to appease these people.
Minni