Hello! I don't think it is possible to revolke a link - the only way I see to prevent someone from seeing/editing a family tree after receiving a link that allows this, would be to save the tree's data and create a new tree based on your backup. However, this means that you have to give everyone you want the tree shared with new links (if your problem is one specific viewer while you want other people to still be able to access/view the tree), and I don't know if contact information saved on the tree (such as phone numbers or E-Mail) can be exported.
Another short note: If your concern really is privacy (and not merely "I don't want someone to be able to edit my family tree"), it is important to be aware that familyecho.com technically doesn't allow to delete family trees - you can "remove" a tree from your account, but the tree itself is still saved and can still be viewed/edited by anyone with a shared link. Should you want to prevent someone from seeing your information, you'll not only need to backup and create a new tree the person doesn't have a link to, but delete the people of the original tree as well.
Hope this helps!
