That would be the Peacekeepers, yes.

And they only don't attack ships without weapons until you attack them to destroy them-at that point they'll attack anything and everything they own.

Yet another reason not to arm your starbases, though.

You don't state specifically, but from your comment on 10 fleets, I assume this is the mega pirate event.

You might try turning mega events off.  The normal "random" pirate event can be cleaned up by 4 or 5 ships, max.  (Okay, maybe 10, spread out through the entire galaxy, if you're playing a gigantic.)

I haven't come across them either.

It is easier to get the Dread Lords event than Peacekeepers.

When the Mega Pirate event happened to me, I didn't really have any ships going anywhere at all (except colony ships, this happened like 15 turns into the game on a gigantic). By that time however, the Drengins had already declared war on me with about 5 transports and 10 defenders (Suicidal difficulty btw), they were literally right next to my only 7 planets and were about to invade, when the pirates showed up, and completely decimated them.

They were begging for peace the next turn (I managed to get Laser I and Planetary Invasion from them, just by going to war with the pirates). Of course a significant amout of other Civs had also declared war on them, and just gifted me a bunch of really helpful techs in the process, so I got into the lead fairly early in that game, and from there on it was an easier win than most.

In my games....

I   am the peacekeepers.

I literraly go about, buying every single one of the galaxie's warships (transports, defenders, you name it, I buy it from them).

AND, I do all this without going into bankruptcy. LOL!!!

I also bribe other civs into signing peace treaties with other civs.

It's a waste of money, but it helps me develop better in the long run.

Isn't buying transports terribly expensive? I've always found that while I can swindle the AI out of a decent part of their warships, buying their transports is even more expensive and often not worth the cost.

Also, you typically end up paying several times more than such warships are worth (compared to producing one yourself and rush-buying it); on the other hand if you sell it to another AI even if it is in dire need of more military and your diplomacy is really high, you'll only get a pittance in return (perhaps 50 times less than what you bought the ship for).

In my most recent game with the Yor.  The Pirates came and destroyed everyone's ships.  2 of the ai's then surrendered to me.  I then went for research victory while the pirates patroled th space lanes.

Isn't this kinda silly, though? I mean, pirates appearing out of nowhere with power and technology that far exceeds what all civilizations can muster together? Why would anyone design such a thing into the game?