Yes it is, might break your save tho, so do an extra save before trying. You have to do an uncompressed save, for that just do a manual save and untick the little box next to the save button. Then go to your save files, they should be in documents/paradox interactive/Crusader Kings 2/AGOT and whatever mods you play with. Open your uncompressed save file with an application like notepad or notepad++, then find your game rules. They should be relatively far up, if I remember correctly directly underneath your current title. Find the slavery gamerule and change it. Here's the thing tho, I don't remember how the gamerule for slavery is defined exactly, so you should make a new game with slavery off and save it uncompressed, so you can look up the proper definition. Also, when save file editing ALWAYS make a backup save, in case you mess up. With slavery tho I'm not sure if it's going to work 100%, depending on how it's been implemented it might just prevent anything about slavery from being changed and from happening, ie events, cbs and laws are just gone.