Using race-wide agents on the Dread Lords doesn't do much good. You should get to at least "low" intelligence on them so you can track their ships, but beyond that it doesn't do much.
However, as Zarnick says, using spies to sabotage them is a whole lot more effective. They never nullify them, and enough spies hammering a DL world makes it take forever for them to build a ship. I would definitely recommend it if you have the agents to spare.
