The Sentinel Frigates emit tachyons. They're too much of a pain to get rid of because they come back, unlike the Tachyon guardians.
Hold on. The fifth time looking at the cheats page, I finally saw the "Military Takeover" cheat. I can use that on the planet that has the last shield generator, and kill it real quick. Alerting another Mark IV planet (not a guard planet) with it will be enough punishment to offset the cheat code. Let me try that.