This feels like a trick question! The boys make up a rule that no one may interrupt the speaker holding the conch. So, technically, the answer would be no one. However, there are many instances when boys, especially Jack (no coincidence, since he is the anarchist figure in the text) break this rule.
The only person allowed to interrupt the speaker holding the conch is Ralph.
“That’s what this shell’s called. I’ll give the conch to the next person to speak. He can hold it when he’s speaking.”... “And he won’t be interrupted: Except by me.”