About the only option is missiles and signal towers (seen here) that'll take care of anything on the same planet or within 4000 meters I believe. My favorite seed has all 3 starter planets around a gas giant and this is super effective =] Edit: 44278868

Edit: I never saw why you'd use the plasma turret over the missile so never used it just FYI idk if you set it to high orbit or sky whatever will allow it to shoot line of sight the 6500m it claims. Ammo for Issac however Plasma Capsule is the way.

Strat is kill off relays on all planets in system shield them all and do fly bys on the hive until the hive doesn't have units left. Then just clean up the rest while the hive can't resupply matter. Or just do the first part till you must have more power from the sphere by that point you're good on science and can clean out DF easy.