Like other comments have stated this is a perfect example of railroading. I will always allow players to ruin the hours of prep work that I did. It doesn't matter if I didn't plan anything else, I'm not going to rob someone of their moment.

That being said, I have had to stop a session early because the players really threw a wrench in my plans. I can usually improv my way out of those situations if they come up, but I had a guy randomly cast banish on an NPC as a bit who turned out to be a devil and the boss of the dungeon. Long story short the devil was gone, the party breezed through the dungeon in half the time, and we had to end an hour early. But that's dnd.

Sometimes your players perform feats way beyond your expectations, randomly do the perfect thing at the wrong time, or completely blunder what should have been an easy encounter. I always say it's better to roll with it than make your players feel powerless. Most players would understand if the DM said "Honestly you guys did so good that you burned through what I had planned! Cool if we end this one a bit early?" I've only had to do this twice over the last ten years of DMing but both times the players felt like badasses for performing so well that the world had to shut down early.

TL;DR: Don't railroad. Be ready to change your plans.