More rock is the cheap way that is somewhat likely to work but will also need maintenance as in more rock or rearranging the rock. Drop structure either concrete, or a pipe is a more sure way of stopping it. Even an under sized pipe with a small dam can work better than just adding more rock. If using a pipe I like to build dam so if the pipe is not able to handle flow it will run around both ends at the same time so as to spread out the flow as much as possible.