Obstruction in Slow Pitch (ASA)

[dsimp8]

Let's say the BR hit a shot to the gap and is going for a double. He runs into F3. I put my arm to signal Obstruction. The runner then proceeds to go back to 1st base while saying he could have made 2nd. I usually drop my arm if the BR does not continue to the next base. In my mind he has given up his right to the next base. Am I right?