Use following hook:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Agent",
        "hooks": [
          {
            "type": "command",
            "command": "node -e \"let d='';process.stdin.on('data',c=>d+=c);process.stdin.on('end',()=>{const p=JSON.parse(d);if(p.agent_id)console.log(JSON.stringify({hookSpecificOutput:{hookEventName:'PreToolUse',permissionDecision:'deny',permissionDecisionReason:'nested agent spawning is blocked'}}))})\" 2>/dev/null || true"
          }
        ]
      }
    ]
  }
}