Am I able to disconnect my immersion heater as i don't need it anymore as i have combi boiler in my home?

Ive have currently got an immersion heater in my home as well as a combi boiler, having just moved in around a month and a half ago i just received my electric bill and gas (both from the same supplier) and my electric bill was sky high and my gas was a tiny �20 for this period so i rang up my provider and asked why it was, so the gentleman from the company asked me to go and read my electric meter again and it came up to me using �60 in 4 days which obviously only 2 adults and a baby cant use that much electric in 4 days (even he thought that was a bit excessive) He then asked me if i had under-floor heaters etc so i said no, he went on to ask if i had an immersion boiler so went upstairs and found yes there was the immersion boiler and he said that is what the cause of the amount of electric usage in my home would be as that was what was heating the water etc and not the combi boiler as it should of been. He said it shouldn't even be connected as we have a combi boiler to heat our home etc. Am i able to either just switch it off completely or would i have to pay to get it disconnected? Any advice is appreciated :)