First you can just simply ask to withdraw your listing from the agent and then if they refuse you can go talk to their broker. The broker may ask if you'd like to be reassigned to another agent in their office.
Personally, I feel that if my client is unhappy with my services and they want to withdraw the listing then I will honor that wish. Because I wouldn't want to tie someone to a contract if they're truly unhappy. I wouldn't want them to feel like they are stuck. To me it's all about good customer service whether I'm working with a seller or a buyer and if I'm not giving good service then I should let them out of the contract. I feel that the agent should be held accountable to the service that they give.
Anyway, that's just my business philosophy! lol! I hope that this helps you with your situation.
Becky Nay, GRI
Keller Williams Realty