Just to check, when you say unable to stop rendering in IR, is the button greyed out? If so, that's likely because a pass limit was set for IR in the Render Settings (it's a separate pass limit than for final render) and the rendering reached that limit and has already stopped by itself (it will start again if you make any changes to the scene, and then you can stop it if you want then).
If the button is not greyed out and shows as still being able to be clicked, but isn't doing anything when you click it, let us know.