What should I do about a collection I don't owe?

A few years ago I had Charter Communication as my internet and cable provider. I had them for about 3 years. It started first as just internet, but after about six months I bundled in cable TV as well. At that bundling they were supposed to have given me the option to buy my router for $20. However, for whatever reason, they did not present this option to me. I didn't learn about this till I went 5 days without any service, upon calling customer service about this they presented the oversight to me. At the time I had been paying rent on the modem for about a year and a half.

I was told they could not refund the monthly fee I had paid for the modem rental. I was annoyed by this but accepted it. At the time I was more concern about my service that was starting to get very poor. There were constant occurrences where either the internet or cable would drop out for days and occasionally both would not work at the same time. I contacted their service department about this and they promised to send someone out to me. After missing work 2 times for someone to show up I was beyond annoyed. I told them about the two absent attempts and they told me that someone would be there the next day guaranteed. The service tech showed up the next day but was not able to fix the problem. He apologized and left. I called the service department again and they said they'd send someone else in 2 days. This time I was given a new box and the service tech told me that it would take a few hours for it to synch. After a full day of waiting it still didn't work. I called again and the sent another tech the next day. This time the tech took the box that the second tech brought and put another box on my TV then left. (He didn't connect it to my TV) I was given the, "it takes several hours of downloading before it will function correctly," line again but I'm pretty sure connecting it may help. I call again, and this time I'm livid. After another day of waiting they finally send someone that knew how to fix things. I was happy to have my internet and cable working, till I got my bill that month. It was $150 higher then it should have been. I called and asked why and they told me I had been charged $50 for every service visit after the first initial visit. I was fed up at this point and told them to cancel my account right then. Unfortunately, based on my timing I owed for that current month and according to them the next month (it's how they bill). I told them they never told me I would be charged for service visits and based on how terrible the service had become I would never use them ever again. I turned in my box and remote and paid the bill up to the month that I had actually used (didn't pay the next month cause I cancelled before it would come up. They told me I had kept equipment so I had been charged for the next month anyway. The equipment in question was the modem that I had bought. After several times showing them that I did pay for it they finally removed that but then told me that I would still owe them $115. Yet they couldn't tell me why. I refuse to give them another dime after all of this.

Recently I got a notice from a credit card that I don't use anymore (haven't touched the thing in 3 years) that my limit had been lowered. The reason why was due to Charter sending me to collection for the $115.

I'm trying to fight back on principle. When I spoke to Charter on the phone they still said I owed for the equipment till I gave them the full detail on the purchase date at which they relented on that but said I still owed the $115. According to TransUnion. My Credit score went from 770 to 715. Also the collection is set to stay on my record till 7/2016. I'm not thrilled by taking a hit on my rating, but I feel I have to stand up to this bullying.

Is there anything that can be done at this point? And if not what should I expect?