They can disconnect you yes dependant on whether they think the T&Cs are being abused. Things that may give them cause to suspect such as abusive calls or texts, using a third party gateway to make international calls or any aspects of the phone being used illegally or fraudulently.
If this isn't the case then you can make a complaint via the link at the bottom of this page.
@Anonymous Are you saying that you were threatened with disconnection if you complained about receiving calls from O2 partners? If you would like to make a complaint, please follow this link: https://www.o2.co.uk/how-to-complain
I don't mean to be rude. I know there are many people on the forum whose first language isn't English, but if you make your complaint in writing you should use a spell checker so that you are correctly understood.