These are the kind of people who make the ridiculous (and false) claim that video games are bad, harry potter encourages satanism, and that a nipslip causes permanent damage to anyone who sees it.
Some of them are nothing but trolls who feed of the resulting media coverage.
Most however actually believe there own insane ideas are correct for example fundamentalists and Allen quist
The appropriate response to these people varies based on situation but most of the time it is one of two things either laugh at them or get angry (but only irritated angry not beat down angry) and point out exactly why they are incorrect.
Normally they have impossibly high standards (religion)
or they have found an axe crazy reason to believe something is corrupting there children.
Normal person: Actually violence per 1000 citizens peaked back in 1982 before doom the first major violent video game was released. For that matter violence has drop by half since then starting directly after violent games started coming into the media.
Moral guardians: Nah, uh our great leader Jack Thompson told us video games is evil.
Normal person: then your a dumbass.