Bought a Apple Watch from someone, and their e-mail is still on watch. How do I factory rest?

I'm afraid that apple don't get involved with lost or stolen or activation locked watches,they can remove the lock if you can provide the original invoice the watch was bought with.

And they can remove the lock online but again you have to prove that you are the original owner.

This is the page;

Additional Support - Activation Lock

The person that you bought/got this watch from needs to follow all the steps in this link;

Remove the Activation Lock so that the buyer can use it.

If they can't or won't then get your money back or recycle the watch and mark it down as a bad experience.

No.

Return it and get your money back,you can't use a watch that has an activation lock on it.