Cool an official Solomani prison planet. INTU, I usually have not planets full of prisons but like Adv: Prison Planet or GT Darkmoon. But, planets full of exiles forbidden to develop infrastructure for star travel.
This way one develops especially in the Solomani Sphere lots of diversity and intrigue. Think Australia, Siberia, New France and America after a few generations. The prisoners strive for respectibility before the whole cycle begins again. It is only the degenerate Imperium that has "Prison Planets", as I believe that they would also have some form of Death penality for High Crimes. (unless you are a noble or rich)...