plugin or product to delete a part of code of templates?

[cloferba]

i have written this on the plugin:

$vbulletin->templatecache['newthread'] = str_replace(
    '{vb:raw forumrules}',
    '',
    $vbulletin->templatecache['newthread']
);

it is executed on newthread_start

but it doesnt work
all what i want is to delete that part from the newthread template