Even if Chaos is not something that can be defeated in the absolute, the Culture would not need to implement something as crude an inefficient as the Inquisition. From wikipedia:

Almost all Culture citizens are very sociable, of great intellectual capability and learning, and possess very well-balanced psyches. Their biological make-up and their growing up in an enlightened society make neuroses and lesser emotions like greed or (strong) jealousy practically unknown, and produce persons that, in any lesser society, appear very self-composed and charismatic. Character traits like strong shyness, while very rare, are not fully unknown, as shown in Excession. As described there and in Player of Games, a Culture citizen who becomes dysfunctional enough to pose a serious nuisance or threat to others would be offered (voluntary) psychological adjustment therapy and might potentially find himself under constant (non-voluntary) oversight by representatives of the local Mind. In extreme cases, as described in Use of Weapons and Surface Detail, dangerous individuals have been known to be assigned a "slap-drone", a robotic follower who ensures that the person in question doesn't continue to endanger the safety of others.
An individual falling to Chaos here or there would not be that hard to contain. And the cause of widespread corruption is usually some external factor, like an artifact or an entity, both of which can be dealt with by quarantine or other methods. Furthermore, it's possible that the Culture's hedonistic tendencies would make it an easy target for Slanesh, but I think one could also make the opposite case. Culture citizens are by and large content. Slanesh has nothing to other them that they cannot already get. And if new forms of mental conditioning conditioning would help, like with the Eldar, implementing them would be fairly easy. Or just copying the natural resistance that Orks or Nids seem to have.