Rob,
Yes there is nothing like "Show module on page".
Instead there are multiple places throughout the templates where code like you have above is used:
Code:
{if $active_modules.Special_Offers}
{include file="modules/Special_Offers/customer/new_offers_message.tpl"}
{/if}
Anywhere that the developer who wrote the module needed to make a template change, they would have wrapped a piece of code in an if statement like the one above. Use webmaster mode to inspect the templates on that category page, it should lead you to the template that needs to be edited.
Since you don't want to display the offers on the category pages at all, you can probably just comment that code block out once you find it.