Just to consolidate everything addressed in this thread:
Card Text trumps the rules. The supplement doesn't really trump card text, but will list card errata and clarify the meaning of text that may otherwise be a little confusing for people. In this case, the Supplement expands upon the rules for spell restrictions, telling us exactly what a restriction like "Forcemaster Only" entails. The rule book already tells us that it means you can't "cast or control" these spells, but the Supplement spells it out so that it's crystal clear that includes prohibiting you from taking control of someone else's spell. This means that a non-Forcemaster can't steal a Forcefield because of the spell's restriction.
With Ability Markers like Runes and Pets we have an interesting situation. No place in the rules or the Ability descriptions for any of these things does it say they restrict the object to a particular type of Mage. Only particular types of Mages can apply them, and they are restricted in that application to creature's that they control. However, after the application of the ability, there are no other listed restrictions that would prevent someone from stealing the object.
Stealing these kinds of objects with special Ability markers sometimes have very weird effects though. Basically, you control the object, but not really the Ability and it will continue to reference back to its originating Mage. Ability Markers are similar to Condition Markers. They don't really have a controller once they're placed, so there's nobody to take control from nor are there any spells that mention taking control of them. Taking the object does not impart to you control over everything attached to that object, and especially not things that aren't objects and weren't controlled to begin with.
Thus, if you steal (Mind Control) a Beastmaster's Pet, it will continue to get a Melee bonus for being in the zone with the Beastmaster (bad for him). If you steal a Priest's Holy Avenger, it will continue to get it's bonus when attacking something that attacked a creature or holy conjurations friendly to the priest that round (good luck leveraging that). I can't think of all the Runes off the top of my head; I don't think this would be a problem for any of the runes, but it is definitely something to keep in mind.
Hope that helps.