Thankfully, though content may be minimized, it's not truly hidden. When such things have happened in the past, users are usually able to "plead their case" to the community and find there is more than enough support to outweigh the bad actors.
The stake weighted nature of voting and the reputation system go a long way toward helping prevent this as well. A bot army has no real teeth to it unless a troll spends a lot of time, effort, and money into building up those accounts so that their flags truly make an impact.