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First place you can report spam = www.bing.com home page. Use the feedback link at the lower right. Indicate the complaint is about spam.
Second place is this feedback form - set it to other and then give careful details. There are other problems you can report here as well.
That above link is the form for reporting spam in search results, or other issues (which you can see in the dropdown menu on the page).
The spam team notes that they especially need on either form:
Hope that helps,
Betsy Aoki, Bing Program Manager
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