how to flush permissions / reset sesson permissions for just one user?

$modx->user =& $user;
$modx->getUser();
 
$contexts = !empty($scriptProperties['authenticateContexts']) ? $scriptProperties['authenticateContexts'] : $modx->context->get('key');
$contexts = explode(',',$contexts);
foreach ($contexts as $ctx) {
  $user->addSessionContext($ctx);
}

  if ($error == false) {
    // no errors and everything was created correctly.
    
    // assign to new role, if applicable
    if($joingroup == 'STD_Members') {
        $user->leaveGroup('STD_Trial');
        $user->joinGroup('STD_Members');
        $user->save();
    }
    elseif($joingroup == 'STD_Members_ID') {
        $user->leaveGroup('STD_Trial');
        $user->leaveGroup('STD_Members');
        $user->joinGroup('STD_Members_ID');
        $user->save();
    }
    
    // Give the correct chunk
    if($chunktype == 'show_success') {
        return $modx->getChunk('Subscription_Change_Success',array('Success_Message' => $Success_Message));
    } else {
        $modx->runSnippet('Session_Refresh');
        return $modx->getChunk('Subscription_Upgrade_Complete',array('Subscription_Type' => $Subscription_Type));
    }
  }
  else {
      if($errors['type'] == 'buy') {
          $url = $modx->makeUrl(95,'',$errors);
          $modx->sendRedirect($url);
      } else {
          return 'An error has happened: <br>' . $errors['Stripe_Error'];
      }
  }

        $modx->runSnippet('Session_Refresh');
        return $modx->getChunk('Subscription_Upgrade_Complete',array('Subscription_Type' => $Subscription_Type));

<?php
$key = "modx.user.{$modx->user->id}";
$keyLength = strlen($key);
foreach ($_SESSION as $sKey => $sVal) {
    if (substr($sKey, 0, $keyLength) === $key) unset($_SESSION[$sKey]);
}

<h1>Success</h1>

<div class="twelve columns">
    <p>You have successfully upgraded your membership to [[+Subscription_Type]]. </p>
    <p>Please wait while the change to your account is applied.</p>
    <p id="countdown">Redirecting in 3 seconds</p>
</div>
<script type="text/javascript">
    var count=3;

    var counter=setInterval(timer, 1000); 
    
    function timer()
    {
      count=count-1;
      if (count <= 0)
      {
         clearInterval(counter);
         window.location="[[~24]]";
         return;
      }
    
      document.getElementById("countdown").innerHTML="Redirecting in " + count + " seconds";
    }
</script>