I think we need to be specific about scenarios here. For example, right now we don't need to worry about extremely high speeds, so defending against inbound objects should be fairly easy. I think for the near future, (US, Russian, and Chinese) interceptor missile technology will outpace abuse-of-spaceships technology, so once there are a bunch of spaceships flying around and/or mining operations get started, it may be easiest to just keep track of relevant objects and shoot them down. I think that should be more than enough, since I just don't think there are very many people who'd want to use spaceships as makeshift weapons who don't have access to better weapons already. If we make it to the far future (where we might expect significant-fraction-of-c speed ships) without singularity things happening for some reason, on the other hand, more authoritarian external control may become necessary.