Well, that's isn't strictly true - utilizing gyroscopic precession, one can generate transnational force via Newton's Third Law of Motion - by that, I mean, since a stable gyroscopic object will resist being turned against it's axis, you can apply force to a spinning, weighted wheel and cause the craft in question to move. Now, it will be an extremely limited movement, and certainly not in a straight line without some serious mechanical linkages to translate the motion. It also would only work in Space, using the Gyroscope as a "fixed object" off which the craft would move. Of course, it would also only work on certain axis of the 'scope. Upon thinking it over more, I believe it would be less an example of gyroscopic precession (the transcribing of a cone by the axis) and more an example of gyroscopic stability and/or angular momentum. Regardless, the amount of motion one could get out of such a setup would be minimal, the devices required overly complex, and the amount of thrust extremely limited (plus, the weight of the reaction wheel needed to move an entire space ship any significant amount would be obscene I'm sure). These sorts of treaties will be meaningless... countries will simply find ways around them (ex - Star Wars Defense Initiative - while the various treaties outlawed militarizing space, none of them included laser based weapons) or, as we are seeing now, ignore them outright (see Donald Drumpf and his "Space Force" military branch idea. I have very little optimism about the future of our species, given the current direction the world is headed.