The stockade and surrounding area was protected by a powerful transporter inhibition field that prevented beaming people to or from the planet. The facility had no walls or guard towers but even if a detainee avoided being stunned by a guard or a droid the planet was devoid of other settlements and there were no sources of water nearby, making escape an unattractive prospect to most inmates.
A number of instructors at Starfleet Academy made it their habit to regularly remind the cadets of the existence of the stockade and how those sent there as inmates rarely wore a Starfleet uniform ever again. (ST - The Fall novels: The Poisoned Chalice, Peaceable Kingdoms)
Following her conviction for murder and treason Valeris was sentenced to life imprisonment at Jaros II. She was held there until 2300 when she was transferred to Starfleet Intelligence custody to assist in an operation to find those responsible for a terrorist bombing over Da'Kel III. Her assistance was crucial in preventing the destruction of Qo'noS, and she was given a new identity and allowed to settle on Sigma Draconis V. (ST novel: Cast No Shadow)
Ensign Ro Laren was imprisoned at Jaros II stockade following her court martial resulting from the events at Garon II circa 2364. She was held there until her release in 2368. (TNG episode: "Ensign Ro", DS9 - Mission Gamma novel: Cathedral)
In 2372, Geordi La Forge compared Admiral Jeremiah Hayes's plans to have him transferred off the USS Enterprise-E to being "transferred" to Jaros II. (TNG - Slings and Arrows eBook: The Insolence of Office)
Ezri Dax was interred on Jaros II following the resolution of the Andorian reproductive crisis. Christine Vale came to see Dax on Jaros II to investigate the incident. Following the revelation that Baras Rodirya had stolen the identity of the long dead Ishan Anjar and the subsequent removal of Baras from office Dax was released from Jaros II and reinstated as Captain of the USS Aventine. (ST - The Fall novels: The Poisoned Chalice, Peaceable Kingdoms)