After moving everything off the planet send a nice note telling the attacker that everything is gone.
I usually use something like; "I hope the battleships are bringing beer because there is nothing left on the planet" or "poor climate conditions have forced all the resources to leave the planet on vacation. There is a 'no body is home' on the door'. 90% of the time they will turn around after sending a probe
You get the idea. Don't antagonize or threaten like Serendipity. Instead, make friends. Most of the people on my buddy list are people who attacked me or I was attacking at one time.
The more friends you make the less enemies you have!
Tenzo