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...