Ssab Swedish Steel Ltd

I hired the National Benefit Authority to help me file forms for a potential disability tax credit. I was unaware that in their clause they state that they require full access to my CRA account. They stated that it costs nothing and they take a percentage of the disability tax credit if I qualify. I never received any disability tax credit and now they have sent my file to a collector stating I owe them $666.01. I have not removed them from my CRA so they can see that I have received no return. I'm worried that it will affect my credit score. I don't want them to have access to my CRA anymore. I didn't understand what I was signing due to my disability, and they took advantage of me that way.