According to page 28:
When a county is in rebellion, the sovereign and all factions present can no longer collect Wealth nor Influence points from the county.The only actions that can be taken in the county are Diplomacy, Espionage, Spellcraft, and Warfare.
The season following the rebellion of the county, the population takes up arms to oppose the sovereign. The maximum number of levies is automatically conscripted, which also reduces the level of the county by 1.
Pages 32/33, Loyalty Adjustment, doesn't include Rebellious.
How exactly a county no more is considered rebellious?
There are two factors at play, here: first, a rebellious province is by definition a hostile province, so should you have cause to roll on that table you would still treat it as a hostile province. Whether such a passive approach is a potential solution is up to the GM; most likely, a GM would insist on a successful use of the aforementioned actions to quell the rebellion. It is theoretically possible to improve the loyalty but still have recalcitrant rebel levy units to deal with via those and other Realm Actions, even if such a passive approach is allowed to raise the loyalty above hostile.
Cool. I thought something like: 1. It is impossible to change loyalty if there is any rebellious unit in the county. 2. After dealing with rebels (killing, negotiating, expelling etc.), then it's possible to change loyalty (and now I see I should consider it hostile).