How can I a revert an overridden template to its default in a sub theme?
I don't know if there is a perfect solution to do that without any inconvenient. Here are two proposals :

Altering the Theme Registry :
To prevent the base theme from overriding a specific template, an idea would be to alter the theme registry. The theme registry array is keyed by theme hook name, and contains informations returned from hook_theme().
As Clive suggested above, you can achieve this by implementing hook_theme_registry_alter(). Here an example of how to revert theme overrides to preserve the original &quot;node&quot; template :
function example_theme_registry_alter(&amp;$registry) {
  // Default template file path we want to use (relative to the Drupal root directory).
  $default = 'modules/node';

  // In Drupal 6, we can retrieve default template path in 'theme paths' array. The 1st key should
  // contain it, however 3rd party code might already have altered the registry...
  // $default = $registry['node']['theme paths'][0]; -&gt; should be 'modules/node'

  // Reassign the template file path and theme path accordingly.
  $registry['node']['path'] = $default;
  $registry['node']['theme path'] = $default;

  // Reassign the name of the template file if needed (could have been overriden too).
  $registry['node']['template'] = 'node';

  // You may also need to unset some preprocess functions to protect template variables as well.
  unset ($registry['node']['preprocess functions'][11], ... , ... );
}

This hook is executed as the theme cache is rebuilt, you will need to clear it to see the changes.
This solution still &quot;allows for overrides&quot; on the module/theme level. However, since this code will always hook after modules and themes, you will probably need to bypass it every time you expect a change (e.g. brought by a new overriding template), just to be sure that what you see is what you expect, and to know which theme do what in the rendered page. So, if allowing module/subtheme overrides while preventing base theme overrides is required, you may want to try the 2nd solution.

Bypassing specific theme overrides :
This is not the clean way but according to your needs and how specific it is, you may prefer to directly &quot;tweak&quot; your base theme :

Templates : prevent override by renaming the theme template file : in the node example, one would rename node.tpl.php to bypass.node.tpl.php (or whatever).
Preprocessing : comment out preprocess functions in the base theme's template.php, e.g: themename_preprocess_node()
CSS, JS, ... : depending on how well your theme is structured, it can be easy or simply horribly boring to comment out the &quot;overriding&quot; part in these files.

The inconvenient here is that you'll have to redo these 3 steps every time you update or change the base theme, but until such a case happens, this will make it easier for modules and subthemes to manage overrides without messing with the theme registry.