To effectively eliminate organisms that undergo chemosynthesis, the most appropriate technique would be to remove methane and hydrogen sulfide. These compounds are often used by chemosynthetic organisms, such as certain bacteria, as energy sources. Removing them would directly impact these organisms' ability to survive and thrive.
Blocking sunlight would not be effective because chemosynthetic organisms do not rely on sunlight for energy. Decreasing temperature may stress some organisms but would not specifically target chemosynthetic processes. Removing oxygen and carbon dioxide would not be effective either since many chemosynthetic organisms do not rely on oxygen and may not require carbon dioxide in the same way that photosynthetic organisms do.