If you can light the burner with a match, then it's most likely burnt on debris blocking/clogging the burner lighter ports (the small ports near the ignitor post.) A small brass (not stainless steel) brush from an auto parts store is aggressive enough to clean away the debris without doing damage. Build up around the ceramic ignitor post can also prevent it sensing a flame and therefore to keep clicking. If it is none of those then the ignitor switch on the burner (behind the knob) is stuck in the "On" position. Sometimes cleaning it will work (the actuator pin on the inside of the switch body, next to the valve stem, is very small and gets clogged with goop easily), but you might have to replace it otherwise.