You have to use a fleet.

If you're strapped for units, putting 2-4 carrier cruisers, preferably well past the edge of the asteroid belt will make a pretty good harassing force for anybody trying to attack your nearby colony. Asteroid belts are a bad place to make a stand however, on the other hand they make excellent early warning sensors, because the only way they can get rid of your station is by capturing it with a Jikara Navigator/Missionary Ship/Colony Frigate

I don't think asteriod belts/space junks etc. can ever be successfully utilised as a frontline defence. You can station a fleet there, but most times enemy fleets will just ignore your units and simply jump on through. Against the AI, I've found that putting a carrier based fleet in one of these places will inflict severe enough losses to allow your planetary defences to finish whats left when they get there.

I've often thought that to make these positions actually worth anything would be either allow you to build some kind of station there, or perhaps more interesting would be an "interdictor" class of ship or Capital ability, to act as a portable PJI. (Although the current usefulness of PJIs in general is still something of debate.)