Sounds like you have an ethics issue.
If the listing agent has not returned a declination of offer letter from the sellers stating that they would not counter or accept your offer, I would ask my agent to have his/her manager call the listing agent's manager to open discussions. If there is still no recourse, please know that all agents have to treat buyers and sellers fairly. All agents should answer offers as soon as they can but no longer than within 24 hrs. If you feel that you still have a problem, you can call the local Board of Realtors and file a complaint...but please know that should be the last course of action.