Most likely, the [ISO] button (on the top of the camera next to the mode dial) is stuck, so it remains depressed even if you don't press it.
Buttons sometimes get stuck, either due to dust or dirt getting in, or because of mechanical damage. I've seen one case where a button got stuck due to a splash of sugary soft drink ;)
Check how the button responds to being pressed. It should have a little "snap" when you press it, if it does not, the button is likely stuck or damaged.
The issue can likely be resolved by cleaning, but this requires dismantling of the camera top panel. There isn't currently a guide on ifixit for the correct procedure, and I do not have a 650D/T4i in my possession to investigate, therefore regrettably I cannot point you to any details.
At this point I'd advise to take the camera to a repair shop and ask for a quote.