Their prayers and supplications would have killed him or made him mad if he let them. He had to cut his believers off or risk loss of mental integrity.
"Mortals can be very stupid sometimes." Stelios shook his head and thought to himself.
That is his first pain-free thought and it is about his remaining believers. The mortals that left him made the best decision to leave. It was either they stick with an unresponsive god and face Helios' anger or switch to another religion that can actually help them. The smart decision is obvious. He would have made the same decision if he were a mortal too. So the ones that decided to be faithful to him are stupid.