Something

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());}