"I'd like to cancel my service."
"But why w-"
"I'd like to cancel my service."
"Buy we co-"
"I'd like to cancel my service."
It's not something I particularly like to trot out, but if I have to spend more than 5 minutes on the phone for something like this, it's what I tend to trot out, and I don't have to repeat myself (and interrupt them) more than two or three times for them to get the point.
"Can you cancel my service?"
"I won't be answering any of your questions, just cancel my service".
"Cancel my service or transfer me to someone who can"
The use of worlds like "Please" and "I'd like to" just tells them that you'll indulge their scripts and tactics. It's not rude to steamroll a system intentionally designed to make it hard to cancel.
I've found a hard stare and a subtle "no" gesture with my head generally does the trick with street solicitors.
The installation tech insisted that he needed to use my computer to complete the installation. My computer (running Debian, so using it was out of the question anyway) was sitting right in front of both of us. I told him that he didn't need to use it (which is true) and he insisted that he did. Then I told him that I did not have a computer. We could both see it, but only by insisting that I did not have a computer was I able to get him to complete the installation without using it (he made a short phonecall, and that was it).