This is a retarded player problem, not a mechanical problem. Any player that runs off alone or 'stalks the party at a distance' is a shitty edgelord, whether they do it horizontally or vertically. Flight can be used very fairly and healthily to reach vantage points or investigate things that are high up. The DM just needs to not be a colossal retard and make sure all his fights have someone with ranged attacks.