Review bombing as a form of consumer protest isn't new, and it's something that platform holders like Valve take very seriously. But the tactic shows no sign of slowing, especially on Metacritic, where the barrier to entry for venting one's spleen is much lower. Indeed, according to an indie developer on the receiving end of a Metacritic bombing, it's astonishingly easy to singlehandedly drag down the metascore for some titles.
Benjamin de Jager from TurtleBlaze, which recently released charming platformer Kunai, writes on Gamasutra that the game has fallen victim to one such singlehanded review bomb, bringing the game's user metascore from 8.1 to 1.7.
"This was no coordinated strike by a group of unhappy players wanting to make a statement of any wrongdoing on our side," de Jager wrote. "Kunai was randomly targeted by a single person claiming to be high and in a state of anger."

De Jager points to a recent (and since deleted) Reddit post by a user claiming to have brought the metascore for Insurgency Sandstorm down. According to a screenshot provided by  De Jager, the Reddit user claimed to have no particular ill-will towards the game, and that the action was prompted by something else entirely (wait for it): anger at the absence of a cherished character in Pokemon Sun and Moon.
I won't post the quote, instead I'll direct you to this screenshot of the Reddit post (the thread, without its opening post, is still available here). While I couldn't verify it myself, De Jager claims that the same user who targeted Insurgency also targeted Kunai.
It wouldn't have taken much to review bomb Kunai: given how few user reviews it has received since launch, bringing the metascore down wouldn't have taken long for someone determined to do it. That's mainly due to the inherent flaws in Metacritic's system: there is no game ownership verification like on Steam, multiple reviews can be posted from the same IP address, and it doesn't block services like Mailinator, which can quickly generate email addresses. In other words: it's easy.
De Jager writes that the studio has reached out to Metacritic about the situation, and that seems to have worked: at the time of writing Kunai's user metascore is back up to 7.4, with only 1 of 16 negative reviews still in the system.
You can check out De Jager's full blogpost here.