I had a hard time figuring out how slaves worked, thought I shared what I observed. Slaves can be a pain with big negative PO effect. Slaves are acquired when you enslave captives after a battle, which can be great for replenishment but can also decrease PO significantly.

Best way I found to deal with slaves is to get rid of slaves is with the sell slaves edict. The edict works well if you control the whole province. If you dont own the whole province, the other faction(s) sharing that province with you could win a battle in their territory and enslave the captives. Since the slaves population is ''managed'' on a provincial level, if your provincial neighbour captures slaves, he will share them with you.

Best way is to deal with this is to TRY owning whole provinces.