There is no inherently easy answer to this paradox. If you want, and allow, user participation, there's no guarantee that it won't be abused in some way. One can imagine all sorts of things that "might happen".
The answer boils down to two things, in my mind: pragmatism, in dealing with real problems, rather than imaginary ones, and tools, that give the site owner some control over the process through which users can participate and upload content.
These two things balance each other: tools can be put in place before "something bad" happens, but you can't know if they prevented abuse or if there just wasn't any impetus to abuse the system. And there's always the possibility that the perpetrators will find some way around the safeguards.
So the balance is in anticipating as many problems as you can, and being poised to deal swiftly with anything that comes up. And, of course, the tools to be able to detect and address any kind of "bad content" that might be feared.
In my experience, abuse is rare, and the benefits of participatory media far outweigh the potential negatives. Even dissenting voices are to be encouraged. It's just those misguided "hackers" that present a real challenge, not unlike the challenge of terrorism: it's almost impossible to prevent terrorist attacks, and perhaps the best way to avoid them is simply to avoid creating a world in which a terrorist feels the need to attack.