...the job was done by the user "himself" or "itself"?

[Pstein]

I am doubting: what should I append in the sentence above if
the user can be woman or men:

himself (excludes women)
or
itself (sounds too child-oriented)

[BuddhaGeo]

"Itself" is mostly used for objects or animals; therefore, it would be more convenient to use "himself" in the sentence you provided.