No, it's one of those double negative things. The way the server is now, creeper explosions are causing huge holes when they blow. Setting this to "true" will prevent creeper explosions from damaging the world. It's confusing, but yeah, it should be set to "true". Think of it like this... "What would you like to prevent? Creeper-blockdamage?" "yes, please prevent that" "... so it's "true" that you want me [server] to prevent creeper explosions from harming/damaging blocks in the world."