Essentially no one. The International Telecommunications Union-Radio is supposed to do frequency allocations but it has no enforcement power except to complain to the home country of a station that's operating out of band and hope they will do something about it. A good example is the 40 meter ham band (7.0-7.3 MHz). Way back in the time of dinosaurs, when I first started listening to shortwave radio, you would hear nothing but hams in that frequency range. A few big broadcasters like Radio Moscow invaded during the 70's and then it became pretty much a free for all, since the 40 meter band offers decent day and night coverage. You can barely hear any hams now because of all the shortwave stations in 40 meters.
40 meters is only a world wide ham band from 7.0-7.1. It is and has been a broadcast band from 7.1 to 7.3 out side of region 2 (the americas). They are not invaders, are operation within the ITU guide lines.
As users transition from HF to Satcoms the ITU is looking at re-allocation of some of the unsed freqs to broadcasting to allow amateur radio to a primary in the 40 meter band from 7.0-7.3 world wide to allow hams to "better provide disaster relief communications".
Actually the 40 meter ham band here in the United States and Region 2 in general are getting easier to operate in. Many of the old super stations of the past have since gone quiet, Radio Moscow is a mere shell of itself in it's many incarnations today. There was a meeting of all the world authorities back a few years ago and it was agreed to not construct new transmitting facilities in Region 1 and 3 countries on the 7100-7300 range, unless there was no other frequency available. Also pre-existing stations have agreed that when it comes time for replacement of transmitters or frequency shifts, they will move out of the segment. Once again this is dependent upon availability of frequencies out of the range.
Forty meters has gotten much quieter in recent years for a number of reasons, which also include better quality receivers with DSP and other filtering techniques. More directive antenna installations from the big gun stations in Region 1 and 3 have aided in this, too. And of course many broadcasters are just shutting down whole segments of broadcast programming all together. This is due to money, and to the governmental shift to more domestic operations in the short range bands, like FM bands. Propaganda transmissions nowadays are limited in scope, compared to what it was during the Cold War years, too. These are still on the air, but very few broadcasters are doing it.
Bottom line, 'enforcement' of HF assignments is by international treaty between the countries involved. A country could, of course, refuse to be part of these interlocking treaties. If they don't cause trouble (follow the layout anyway), no one would care a lot, probably. If they DO cause 'problems' (jamming, whatever), you're into the whole international power game. Do we send them a 'stern note'? Impose financial santions? Do we bomb the interfering station as a 'subtle hint' of displeasure? Depends on the country causing trouble, and the one feeling annoyed.
Time was when FCC and other nations (England for one, via BBC) operateded Monitoring Stations that routinely prowled the spectrum for interlopers as well as rules violations. The one us Midwesterners used to watch out for was in Grand Island, Nebraska. In addition, the guys from the regional FCC officers, while moving around covering paperwork and such used to monitor locally too. It kept all of us a bit more honest and interested in complying with the laws.
Internationally, proven and outright violations of International Spectrum allocations and treaties are largely officially complaioned about by the State Department and FCC through letters. And, that has little clout these days either.
It once was a great time to be interested in radio purely for radio's sake. I'm glad I got in on some of those great times circa 1930s-1950s. A whole different kind of fun and sense of accomplishment.