star trek - Who is bound to the Prime Directive?

In the galaxy there are a fair number of pre-warp societies. A Federation ship is not to contact them. But what about other, non-Federation species?

If for example a Ferengi ship passes by a pre-warp planet, would they be allowed to contact it? What would a Federation ship observing this contact have to do? Are they obliged to intervene? Are they allowed to? Or were they merely allowed to "strongly discourage" the attempt of the Ferengi?

If it is legal to use force to stop the Ferengi, than in essence the Ferengi were bound to the Prime Directive too. And if so: does this go for all Federation territory? Is this confined to the Federation territory? If Janeway observes such an incident, what are her rights to intervene? Or does she have the duty to do so?

Answer

Looking at the high-level summary of the Prime Directive from Bread and Circuses:

no identification of self or mission; no interference with the social development of said planet; no references to space, other worlds, or advanced civilizations.

So it tells Starfleet personnel to refrain from interfering themselves in the development of other species. It doesn’t tell them to prevent interference by others — and, as @Selezen points out, preventing such interference could be thought of as interference in itself. (Starfleet might not exist if the Klingons had had a pro-active version of the Prime Directive and stopped the Vulcans from popping down to Earth for a cup of tea with Zefram Cochrane).

I believe the intention of the Directive is to stop Starfleet from playing god:

I'm going to have to remind myself every day that we didn't come out here to play God.

Captain Jonathan Archer (ENT: "Dear Doctor")

As such, it’s ultimately intended to protect Starfleet from behaving like Q (toying with less-developed beings to satisfy its own curiosity), more than it is to protect lesser-developed civilisations from any interference. Starfleet therefore does not generally attempt to enforce it on other species, or even non-Starfleet Federation citizens.

(Note that the Federation is an interstellar federal government, whilst Starfleet, to whom the Prime Directive applies, is its military/exploration body.)