Yes, it is possible to turn off the Orion Pirates.
If you go to your starfleet command 2 folder, have a look in the MetaAssets/SinglePlayer folder. There you will find a file called MetaMap.gf, you can edit this with notepad. In the file, find the entry (about half way down) that has:
OrionPopulationRatio = 0.15 // (0.15) This is the population ratio for orion pirates.
You can change this to 0, so that no Orion Pirates will appear on the map.