Cancel a subscription or contract that won't let you go

I want to cancel something and the company is making it difficult.

What I'm canceling: Home internet, 3 years with the same provider, out of contract since March
What's making it hard: There's no way to cancel online. The phone line puts me through to 'retention' who keep offering discounts and won't process it. They also said I'd need to return the router within 14 days or pay $150.
When I want it to end: the end of this billing month, September 30
Where I live: New York, US

Give me:
1. A written cancellation notice. Under 120 words. States that I'm canceling, the date I want it to end, that I don't want a retention offer, and that I want written confirmation with the final bill amount. Something I can send by email, chat or letter and keep a copy of.
2. A short phone script for when I get put through to the team whose job is to keep me. What I say first, what I say when they offer a discount, and what I say if they claim they can't process it.
3. A list of what to write down during any call: names, times, reference numbers.
4. What to check before the final bill, such as equipment to return or payments to stop, based on what I've told you.

Don't tell me what my legal rights are unless you're sure for where I live. If there might be rules that help me (on canceling online, or fees after a contract ends), tell me what to look up and which official body would know, instead of stating them.

What you get: A written notice to keep on record, and a script for the call where they try to keep you.