Quaternion orient = cameraNode->getOrientation();

if (orient.getPitch() > yourLimit)
{
    cameraNode->resetOrientation();

    // Limit your pitch
    cameraNode->pitch(yourLimit);

    // Re-set yaw and roll (I think you can omit roll, since it's not used in quake style camera)
    cameraNode->yaw(orient.getYaw());
    cameraNode->roll(orient.getRoll());
}