1. SOME premade maps done have pirates... (you can look at pirates screen and see if they start out with a timer or with a shattered glass meaning they are dead).
2. You can make them into much XP for your capital ships by killing them.
3. Bidding makes them MORE POWERFUL. The pirate fleets get more powerful (SIGNIFICANTLY SO) the more money was put into bounty.
4. Make a choke point, they will always attack a colony, and will never fly by ONE colony you own to attack another...
5. Fortify the planet, they leave as soon as all orbital structures are down (to joing the defense fleet of the pirate home, so be careful about letting them leave). If the planet survived the bombing up till all the orbital structures and vessels are dead then they will leave it alone and alive.
6. If you have a trade port then half will go chase freighters around the system while half will attack it. If you put it next to the planet with a bunch of turrets then the turrets will shred the bombers, and then shred the attackers. Repair stations will repair the trade port AND the turrets... and once they die the ones chasing freighters will eventually wonder close enough to be killed... Or if you have a hanger bay then the fighers and bombers will kill them (each pirate ship is weaker against another of the two, so get half and half)
7. Kill the pirate base... Colonizing gives you the "pirate booty" which adds 3.5 or so credits per second (if you settle the pirate base) and stops all attacks.. I have also found a major artifact there once.
HOWEVER, if you send a bunch of capitals they will be SHREDED!
An army of frigates and cruisers is MUCH more powerful then capital ships.. however it dies quickly and needs constant replacement forces, and costs MUCH more... but for pirate bases its the only way to go... get a large fleet of at least 50 frigates or 25 combat cruisers... AND send the capitals in as well... (to get the XP)... you might have to level your fleet capacity to build all those ships...