This will prevent PMs from being quoted in replies however, as there is no 'Reply with quote' option in PMs this will remove the option to quote completely unless a user copies and pastes the original text into the reply.

Change 'newpost_quote' to read as follows:

Code:
<vb:if condition="THIS_SCRIPT != 'private'">[ QUOTE=$originalposter]{$pagetext}[/QUOTE]</vb:if>