By default, attachments should not be visible in the library. The "Add document to the document library for others to use" is out of the box unchecked by default. I'm curious if your organization has changed the default for some reason. Most attachments are specific to the record in question (especially purchasing documents) so we typically leave this as the default. If you want to adjust this on any records, this is stored on the DOCINFO table SHOW column. You can update these to 0 and they will no longer be visible in the Manage Library dialog. If this is defaulting to enabled, you should also check the DOCLINKS.SHOW attribute in database configuration to see if that is set to 1.
Assuming it needs to be available in the document library, you could use data restrictions (typically we use global data restrictions which are managed in the Security Groups application) to restrict what is shown in the library. We've locked down certain folders that way before to hide certain documents that only some groups should be allowed to see. You would have to get users to reliably put the attachments in that folder for it to work, which is really difficult.
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Steven Shull
Projetech Inc.
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