To effectively eliminate organisms that undergo chemosynthesis, the best technique would be to remove methane and hydrogen sulfide. Organisms that rely on chemosynthesis often use these compounds as energy sources. By removing them, you would disrupt their primary mode of energy production and severely impact their survival.
The other options, such as blocking sunlight or removing oxygen and carbon dioxide, would not effectively target chemosynthetic organisms, since they do not rely on sunlight or traditional photosynthesis. Decreasing the temperature might affect some metabolic processes, but it would not be as effective as removing their energy sources.