There is a Chinese term 网路水军 (wǎnglù shuǐjūn) for people paid to attack a particular target on the Internet. I've seen "spammers" as the English for this expression, but to my mind a spammer usually operates by sending email or texts in bulk or by robo-phoning. In contrast, I'm interested in finding a term for people who specialize in posting bad reviews on commerce sites or discussion boards.
Example: "My company hired ________ to smear its competition in on-line reviews, but we had to apologize after being found out."
Update: There's some interesting discussion going on in the comments and subcomments. Good people, please be gentle with each other.
I'm not necessarily looking for an outright translation of the Chinese term I mentioned; ideally, I'd like to find a pre-existing term in English. The phenomenon of fake testimonials or criticisms of a product, in Anglophone society, goes back a long way before the Internet, and I don't think water armies would be comprehensible to anyone not already familiar with the Chinese term.
I'm interested in exploring sock puppet, which I've seen used to refer to the poster of a fake review or up-voter on Stack Overflow. The OED has a year 2000 citation to a related figurative usage, which it defines as "a person whose actions are controlled by another; a minion." (The citation calls the Presidential candidate Al Gore a sock puppet, meaning that he is just a front for some more nefarious political force.) Slang often has fluid meanings, and I'm not sure sock puppet has really settled into the meaning I'm looking for.
But I'm learning a lot from the discussion.