When I played the base game I would use Warfare I to trigger a second invasion, but now that I am using Shatterd Empires and Shard of the Throne I don't see how to deal with this pesky leader.

Perhaps there is an action card that can help, but that is not very reliable. Can you used the bio weapon tech to kill all GFs?

What would you do?

Diplomats can be a nuisance, but they can be removed. If you can turn the planet neutral somehow (such as with a well-timed Local Unrest), the Diplomat will flee the planet.

However, the important thing to remember is this: Once a Diplomat has used his power to block an invasion, that planet cannot be protected again by ANY diplomat for the remainder of that round OR the next round.

So if a Diplomat protects a planet, and you use something like Unexpected Action or Warfare to remove the CC, you could try to invade again, and this time the Diplomat won't help. Or if you wait until next round, the Diplomat won't help either. (If you wait until the round after that, it can work again, though).

It can also happen where if Player A has a Diplomat on a planet, and Player B invades but is blocked, Player C could come in, wipe out Player B's fleet, and then invade without being blocked.