Web Player Disable Mouse Look

Is there a way to disable the mouse look when the cursor leaves the web player area on a web page?

Sure. Why not?

You could write a script on your page which tells your web player to enable/disable the mouse look.

You could check your Input.mousePosition and if it is outside of the 0,0,Screen.width and Screen.height and disable your mouse look when it is or enable it when it isn't.