How can I stop player from using a piece of furniture but allow NPCs to use it? In this case an oven.

The player can just not use it, since you are the player and are in control of the avatar. Not sure I understand your dilemma.

Don't give the furniture a name, so the player cannot click on it and activate it.

alternatively, if you wish to give it a name, add the defaultBlockActivation script to it.