This clearly refers to a user account on an Internet forum.

"Usuario" here is not just the username; it includes all the details, including email address as well as personal data, needed to create an identity with which to open a user account under an assumed identity. In English it is quite common to say "create a fake user", but this is Internet jargon, and in a legal document I think a more formal expression would be used: "false" rather than "fake", which is too colloquial, and "user identity" rather than just "user". "User account" would be OK as well; you could argue that "usuario" is both the identity and the account.

Here's an example of an English legal usage:

"Lori Drew faces federal counts of Conspiracy and of Accessing Computers Without Authorization. Drew is charged with creating a false Internet identity on the social networking site MySpace, posing as a teenage boy."
http://www.burneylawfirm.com/blog/category/computer-crime/

For "calumniar", which is specifically to commit calumny, that is, falsely accuse someone of a crime (see earlier question), there is no verb "calumniate" in English, and I think we must use "libel". Defamation on the Internet counts as libel, not slander, because it is written and published:
http://en.wikipedia.org/wiki/Defamation

Finally, "injuriar" is a "falso amigo"; it does not mean "injure" but "insult".

By the way, "libeling" is spelled like that in American English, but "libelling" in British English.


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Note added at 1 hr (2012-03-15 02:52:37 GMT)
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Sorry, but on reflection I don't think "insulting" is right here, and the present participles don't work well in context. "Injuriar" can mean to slander someone, but as I say, slander is spoken and comment on the Internet is not, so that word should not be used. "Injuriar" is something that damages someone, and despite what I said before "injure" is used in this sense. However, I think the best translation here would be something like this:

false user identity created for the purpose of spreading libelous and damaging falsehoods

"Libellous and damaging" is a very common expression, sometimes with "malicious" as well, used here, for example, by the US Supreme Court:

"defendants charged with expressing libelous and damaging falsehoods in petitions to Government officials"
http://bulk.resource.org/courts.gov/c/US/472/472.US.479.84-4...




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Note added at 1 hr (2012-03-15 02:53:59 GMT)
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Or libelous and damaging statements.

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Note added at 1 hr (2012-03-15 02:56:09 GMT)
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But better making libelous and damaging statements. (Sorry to keep changing my mind.)

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Note added at 1 hr (2012-03-15 03:04:27 GMT)
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I far prefer this formulation with adjectives. There is no adjective "calumnious" in English, but the sense of calumny is conveyed by what follows: "la denuncia de hechos falsos". "Defamatory" could be used, but I prefer "libelous" ("libellous" in British English, by the way).