Please reference the Codex entry for wp_list_comments().
This function includes a 'max_depth' parameter in its args array.
On the specific page in question, simply call:
<?php wp_list_comments( array( 'max_depth' => '1' ) ); ?>
If you need more specific help, please clarify your question to indicate how you would identify the specific page in question. By title/slug/id? By Page Template? Some other way?