Not really my nemesis but I once encountered a fellow human being who 'stole' my spot on a gym. He didn't do it intentionally though. It took me a while to heal my Pokémon so there. Long story short, he's my pal in real life now
Edit: I know you probably mean a player on another team. However; I live where gym turnovers happen like after 30 mins so I can't really tell who undo my hard work. Though I remember the same old IGNs who always pop up in my town.