First of all, it will be in your best interest to approach them in a professional manner in regards to voiding the contract. The major issue that you might face with this is that you might be financially obligated to them regardless of who may purchase the property. Hopefully your contract is a short one, say 3 months.
Next thing to do is to contact the broker who she works for. If she is making your life difficult, then let the broker know. Most professional brokers will approach the situation with wanting to make you happy.
If this takes assigning another agent, then that may work. The broker would be able to void the old contract, so make sure that it is.
Also, note that there is a date associated with the contract ending that extends the date further out, for closing and etc.
Best of luck and hope that it works out for you...
You need to speak first with the agent. If all she did was bring a family member to view the house, that may not be cause to dismiss. Maybe they are legitimate buyers?
If you cannot resolve your differences, speak with the agents Broker and asked to be released from the contract. Your contract is with the Broker. The broker will likely attempt to assign it to another agent. Just stick to your guns. You will get it resolved.
Ask your agent to be released from your buyer's agreement. (It's a contract, so both parties must agree.) If she declines, then go to her broker. Ask her broker to release you from the buyer's agreement with that agent. Technically, the agreement is actually between you and the broker. As a result, the broker will very likely want to assign someone else from the same office to represent you. And that's OK--just interview the prospective agents before selecting one.
Hope that helps.