You can try to deliberately get follower quests stuck, I guess, simply by avoiding whatever they're asking you to do. I'm not sure how foolproof that is though, because sometimes your companions will simply take over or, if you don't let them, start hating you and eventually leave. This is compounded by the fact that oftentimes key characters in your companion quests are also key characters in the main quest chain. Furthermore, the game sometimes tries to 'help' and 'unstucks' quests by allowing you to skip certain quest steps... in short, if you really want to avoid getting companion quests spoilered I would probably suggest leaving said companions out of your party entirely.
I really understand how you feel but I don't think the game gives you enough control over this. I mean that's part your companions too right? It wouldn't feel right if they just stood next to you chatting up their arch-nemesis and went 'This is FIIINE.'
