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This book writes to say
In 1981 the USSR requested and was allocated seven satellite slots in 24-hr orbit for Earth observations, calling the system Prognoz
Geostationary slots are a rare resource and are assigned to countries by the ITU (International Telecommunications Union) since they are mainly used for communications and broadcasting, and need deconfliction both in frequency allocation (to reduce interference - the main raison d'être of ITU-R as a division of the ITU) and in collision avoidance (tighter orbital boxes are costly in terms of lifetime xenon expenditure).
Almost everything else in orbit selection is based on the operator's discretion, provided the downlink and uplink frequencies do not conflict with other satellite, fixed and mobile services.
Orbits are checked through national space control centers, though, to avoid crossing paths with other satellites and space debris.
For more information on Geostationary services, please have a look at http://www.itu.int/ITU-R/space/snl/
EDIT: On sanctions and reprisals: violating an international treaty is bad since the culprits will find themselves shunned. States do have the ability and legal authority to jam or disrupt non-authorized transmitters that broadcast into their territories, so it shouldn't be surprising to the perpetrator to find his downlinks or uplinks jammed. It should be noted that GSO is far away from the Earth, and the link budgets are quite slim, so any modest interference will degrade the link beyond usefulness.
TL;DR - A non-state actor (a corporation) will be held liable by its national telecommunications (and space) authorities, while a non-compliant state will be unable to effectively use the slot in GEO due to easy counter-measures.
The usual disclaimer: I am not a space lawyer...
To add to Deer Hunter's response:
The ITU manages the radio spectrum that satellites use, not their physical location.
There are many examples of satellites being owned by different organisations whose ITU permitted orbital slots are exactly co-incident but for use with different frequency bands. It is then up to the owners of the satellites to avoid physical encounters.
There is a further detail which is that a satellite does not actually need ITU permission to use certain spectrum in a given orbital slot, and this is recognised by the ITU, if it does so on a "non-interference basis", i.e. taking their own care not to cause radio friequency interference to neighbours who might already have registered frequency rights.
There has been an interesting related case in the news very recently, see http://spacenews.com/russian-satellite-maneuvers-silence-worry-intelsat/ , in this case Intelsat had become concerned about the presence of a Russian satellite physically close to their own satellites. The article describes their concerns about the difficulties in establishing contact with the Russian satellite control authority but nowhere is there any mention of ITU permissions and, I suspect, this is because there is not necessarily any breaking of those particular conventions.
It seems this is at least partly handled at the country level, as Swarm, a US based company was recently fined by the US Federal Communications Commission for unauthorized orbits that apparently risked satellite collisions.
