In the site's <code>wp-config.php</code> file, define a constant to disable output of the plugin-defined JavaScript: <code>define( 'C2C_ADD_ADMIN_JAVASCRIPT_DISABLED', true );</code>. You can then visit the site's admin. Just remember to remove that line after you've fixed the JavaScript (or at least change "true" to "false"). This is an alternative to the query parameter approach described above, though it persists while the constant remains defined. There will be an admin notice on the plugin's setting page to alert you to the fact that the constant is defined and effectively disabling the plugin from adding any JavaScript.