terminating an existing petition?

When I needed to cancel a petition, I didn't use any special form, I just wrote a letter to my local immigration center where I sent the petition and asked for it to be cancel including the reason why. They never reply but they did cancel the petition.
 
I haven't done one but I was told by an immigration officer during my visit to DO that I have to write a letter explaining the reason and they will terninate it. She also said that I will receive a confirmation on that.
 
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