Turning off the sun, as in permanently extinguishing its fusion reactions, would require overcoming the immense gravitational and nuclear forces at play in its core. The sun generates energy through nuclear fusion, primarily converting hydrogen into helium, and it's this process that keeps it shining.
Removing its fuel: Extracting a significant portion of the sun's hydrogen. However, considering the sun's mass is approximately 1.989 × 10^30 kilograms, removing enough hydrogen to halt fusion is far beyond our current technological capabilities.