Who's Going to Police Space?

I don't usually see malicious plots at every turn, but this phrase from the BBC's Spaceman blog caught my attention:

Our customers are principally two major groups. The first group is sovereign clients - other countries that do not have the kind of access to the ISS that they would like to have, and that want to shape their own space futures.

So when space access, which is largely controlled by governments at the moment, switched primarily to the private sector, who stops just anyone putting a weapon in space?

I'm not talking nuclear warheads, lasers or giant mirrors, the simplest weapon would just use gravity for kinetic bombardment. No moving parts, no exotic materials, no real know-how, just let it go at the right point and it can be dropped anywhere on Earth and is impossible to stop.

So who polices space?