How to lock a player on an axis so they don't go off the path?

I am currently working on a side-scrolling and I want to lock the players on the z axis so they cannot move off of the path, I already have it so when the player presses s it does nothing and when they press w all it does is make them jump, however objects on the path or jumping certain ways can make the player get knocked off a bit. Is there a way I can keep them on the path I want them off that does not use invisible bricks on either side of them? (Messes with a tool's aim in the game.)