I received a call from the some Buyers. They were referred to me. They had been looking at houses on an off for a few months. Whenever they wanted to see a house they would call from the info. on the sign. Now they want an agent to do CMA's and get property history on some of the properties they looked at. I asked them if they had been working with any other agents or had a Buyers Agency Contract and they stated no. They got preapproved and I gathered the info. they needed to make a buying decision. After a week and a half I we decided to make an offer. After faxing the offer I received a phone call from the listing agent stating that I had "stolen" her buyers. She said she had shown them the house. She also informed me that she had spent alot of time and money on listing this property and her commission would be minimal. She asked me to void the contract. She contacted my buyers and told them she felt as if she should represent her. They explained to her that they wanted me and that I had helped them come to their buying decision. My buyers were ready to retract their offer and find something else. I told them to continue on and we would work through this. I received unprofessionalism from the listing agent all the way through closing. She has now filed a complaint with the Realtor's Board. Did I do the right thing?

I think you did, but you should have asked for "buyer agency" to make sure there are no problems down the road.
Even at this point I would ask the buyer to sign a buyer agency agreement. (if that's possible)

They will probably try to get you on "procuring cause"
see www.realtor.org

There is a whole arbitration worksheet available on realtor.org, it's not always about who showed the property first. As a seller's agent, I believe that it is my duty to my sellers to show the property and give out information - even if the client has an agent already. My job is to sell the property - not just try to double end it! Best to get a Buyer Broker Representation Agreement, if not before you show any property, then when you are writing the offer.