Re: This is not a problem for asteroids
A 1000km asteroid would have a significant gravity well compared to smaller ones. You need to economically get your material off it, especially if you aren't actually processing it in situ but are moving it elsewhere for easier processing (i.e. somewhere closer to the Sun where solar panels will work better than on Ceres)
Maybe you'd need to establish some rules about the sizes, or named bodies or whatever that can't be so claimed, but I think it is unlikely anyone would want to claim Ceres if it meant they had to actually mine it for real, not just stick a lander on it and claim they are "exploring". There's no reason to expect that Ceres is unique in the quality of its ore, and its size/gravity makes it likely the best ores are deep under the crust as they are with Earth (but unlike Earth, volcanoes aren't going to bring the gold from the deeps to the surface) Ceres is probably the wrong rock to set up on.