To unshare an artifact or modify a user's sharing permissions, first open the artifact you'd like to unshare or modify. Once there, click on the orange 'Share' button.
On the right side of the sharing modal, you will see all the users/observers who have access to the artifact along with their permission level.
To modify the user's sharing permission level, click the pencil icon next to the user's current permission level.
Then, select the level you want the user to have.
To unshare the artifact, click on the trash icon all the way to the right of the person's name.
You will then be asked if you are sure you want to remove this user's access to the artifact. If you are sure, click 'Remove Permission.'