In an emergency, if you have no valve in the sewer line, or if the valve fails, you can use a plastic bag filled with rags to stop back-flow through the sewer.
The pictures below show the bag being placed in the sewer line at the sewer clean-out. This clean-out should be accessible in the yard, between your house and the point where your sewer line enters the municipal or private sewer system collection line.
You can also use the bag-o-rags technique inside the home. To block back-flow at the toilet, remove the toilet and stuff the bag of rags in the exposed sewer pipe opening. Do not stuff the bag of rags into the toilet through the toilet bowl. This protection is similar to what you get using gripper plugs and involves the same amount of work on your part.