Actually, the average FM pirate is shut down due to *a* complaint - interference oftentimes doesn't factor into the equation at all. Read the Notices of Unlicensed Operation handed out by the FCC - they typically say, "this field office received a complaint of an unlicensed station on frequency x." That's all they need to begin action: a pirate's very existence is the violation.
What the FCC says is usually boilerplate; what I was talking about were the n00b operators who didn't get a filter and their neighbors called the radio fuzz because the pirate signal was coming through their TV, and unfortunately you see more of those guys in the pirate FM field*. I know all it takes is one complaint to start the ball rolling, it's just where those complaints come from is important.
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* And in CB; I grew up in an area where there were two 11 meter machomen with the dirtiest linear amps in history - if you tried watching a faraway station and they keyed up, the picture went to static and you could hear their conversation.