Brennan is right, they won't attack you as long as your ships don't have weapons. If you want to exterminate them, research down the Missile weapons branch of the Tech Tree. They are Beam weapon based, so research down the Shields branch if you wish.
And I would love to have Pirates for once attack my galaxy. I've never faced Pirates. I would love to go pwnzaur all over their sorry candy-asses. It would be a nice test of my skills.
Also, your starbases will come out unharmed, as long as you didn't upgrade them with weapons.
In one of my games the Peace Keepers appeared the turn after the Dread Lords appeared. It was quite epic, Peace Keepers and Dread Lords duking it out. Of course it turned out as a stalemate due to the fact that Dread Lords are Missile and Chaff based, and the Peace Keepers are Beam and Shields based.
Eitherway it was funny and I was able to mop up quite well.
Hope this helps.
Rock On!!!