The same guys who bitch about the street peddlers will espouse the benefits of a free market economy over cocktails.
I've been happy that some (not all) of the street vendors sell their wares in places where mongers congregate. I've bought an umbrella once, and a pair of really crappy sunglasses...but they did the trick until I was able to get back to my room and grab my good ones.
V.B. has much more experience with this than I, but I've always found that a polite but firm "no" will usually be enough. If they persist, my phrase of choice is "Dejeme en paz, por favor." They're surprised at both the Spanish and the politeness.
What they could sell to service mongers:
1. Breath mints and gum
2. Mace or pepper spray
3. Condoms
4. Maps to tico bars with chicas
5. Spanish/English phrase books
Well, it's just an idea.