You can flag the job as inappropriate, and give "Asking for free work" as the reason. But you sent the client samples without a contract, so you won't receive a penny in return.
John B wrote: 'I would like 3 blog posts(articles, whitepaper) that would be relevant for our website...'
If you obliged, meaning if you provided those 3 articles as part of the interview, which means the client was yet to raise a contract, I'm afraid you have no recourse.
Next time ask the client to raise a contract for the test article. If you look at profiles of seasoned contractors, you'll see tiny amounts like $20, $10 or even $5, which constitute pay for test work. Alternatively, provide the client with a sample of past work, selecting that part that you think will portray the skill or style the client is interested in. Personally I even provide a paragraph or two from the middle of a book I've written just to show my style.
So, you can go on and report the client as a scammer, but not with the hope of being assisted to get your work paid.
Please, make sure you have a contract with the client before you start doing any work for them. That way you can benefit from Upwork Payment Protection and receive your payment. If the client requests you to work on the assignment without a contract, please flag their message or original job posting.
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