Once I decided to end my contract with Virgin Media and they kept asking me why I was leaving, so I kept saying I didn’t want to explain, I’d just cancel (because I knew they’d do their best to talk me around) and it got to the point where I became firm, but I didn’t shout, though I wanted to.
Tangentially related: after I’d signed up to the Telephone Preference Service, I knew that the only people ringing me to sell stuff were doing so illegally, so if they persisted after I’d made that point, I used to just verbally abuse them. Right cathartic.