Have there ever been any cases of any countries or constituencies laying claim to any space territory? Is there any legal framework in place for such a claim (or indeed, to nullify such claims?)
Although this may be surprising, there is a whole discipline called "Space Law". As there is space law, there are space lawyers. You should look into the work and activities of e.g. http://www.virgiliupop.com/ (link broken as of 2021)
There are countless private claims on territory in space. The maybe strangest one is this: https://en.wikipedia.org/wiki/Nation_of_Celestial_Space
You might want to have a look at https://iislweb.space/
As for the exact legal regulations, have a look at https://en.wikipedia.org/wiki/Outer_Space_Treaty The current philosophy is, that space is common ground and, as it is "extraterrestrial", no terrestrial nation can claim territory in space.
EDIT: As stated in the comments, it is worth mentioning that there are analogies with the protection of Antarctica. For a good introduction, have a look into this paper: http://si-pddr.si.edu/dspace/handle/10088/16170
According to Who Owns the Moon?: Extraterrestrial Aspects of Land and Mineral Resources Ownership by Virgiliu Pop, people have been attempting to say they own lunar or Martian real estate since the 1890s. For example, A. Dean Lindsay of Ocilla, Georgia attempted to say he owned tons of extraterrestrial real estate. However, none of these purchases of extra terrestrial territory have any legal standing.
This information is also available on Wikipedia (which references the book as well) at http://en.wikipedia.org/wiki/Extraterrestrial_real_estate .
A chilean called Jenaro Gajardo claimed ownership of the entire moon back in the 50s. The legend says that even President Nixon asked him about the Apollo XI mission but I have not found any reliable source for the latter.
In the Bogota Declaration, several equatorial countries laid claim to geostationary orbit (which lies above the equator). They carefully (if not carefully enough) called this a resource rather than territory.
This claim has been ignored by those nations and organizations that put satellites into geostationary orbit and by the international organization that allocates satellite positions within geostationary orbit.