Villager XP can not be disabled.

Use the following command:

/summon Villager ~ ~1 ~ {Offers:{Recipes:[{rewardExp:0,maxUses:10,uses:0,buy:{id:cobblestone,Count:1},sell:{id:writable_book,Count:1}}]},NoAI:1}

rewardExp should disable the experience from villagers, but it doesn't seem to work.