The fusion just quietly stops (side note: if you got that reference, then it’s time to go outside). There’s a lot of loose talk about the Sun being a huge atom-smashing machine, but in reality the hydrogen that fuels the Sun can wander about in the core (where the fusion takes place) for several hundred million years on average without being used in fusion. The rate of fusion that takes place in a fusion bomb, or that we hope to accomplish in fusion reactors, is more akin to an exploding star than a stable one. The core of the Sun (which is huge) produces heat through fusion at about the same speed that the human body produces heat. A good ‘n scary sci-fi weapon wouldn’t be something that halts fusion, but something that directly cools stars suddenly, or goes the other way and speeds fusion up (which is the important step in a nova).