You can't directly, but you can turn off the "auto-leveling". As you might have understood, the universe is mapped out to just a 2D plan, much like our own Solar system is like a disc.
If you turn off the auto-leveling, your ship will not anymore try to turn itself correctly to the right "up" position. Normally in the game you have destinct feel (even if a tad unrealistic) for what is up and what is down... Closest comparison I can make here is Descent, where the game was in 3D, but you also there had a feeling for where up is.
So sometimes in combat I turn auto-leveling off, since I'm in a "fur-ball" of enemies, and I don't want to wait for the ship to roll to the right direction before heading for the next enemy. After combat I often turn it on again to not loose my orientation.
Best Regards
Christian "Bargib" Koerner
Editor in Chief,
The Lancers Reactor