My group ran into a problem today, from a literal reading of the ruels how do you banish a spirit that has gone rouge. Since it doesn't owe anyone any favors doesn't that make impossible to banish?

There seems to be no rules for free spirits in the main book. Just the "uncontrolled spirits" box that simply says that the spirit will try to kill it´s master if he looses control.

Banishing is now specificly the process of severing the tie between summoner and spirit, on p 180. By a litteral reading of that page I see no reason why a banished spirit wouldn´t stay and take it´s revenge. That does of course contradict the term "banish". On page 113, the skill Banishing is said to "disrupt spirits, removing them from the physical planes".

I would take the skill description as an indication that the rules on page 180 detail how to "disrupt and remove" summoned or bound spirits. In case of summoned and bound spirits, all it takes is to excuse the spirit from service and let it go home for the day. It´s not that strange that the rules only cover those kinds, because free spirits are only mentioned in a black optional rule box.

I think it would be reasonable to assume that you need to deplete a free spirits complete force to bannish it. Maybe even force+spirit energy (if they´re gonna keep that term). I would let spirits use it´s magic or force in place of the summoner´s Magic also.

Just hack the damn thing to bits in the meantime.