Odoo 8 (Technical menu grup):

Settings -> Database Structure -> Attachments

Openerp 7 / 6.1 (Technical / Customization menu grup):

Settings -> Low Level Objects -> Attachments

Then you can pick any records on the list, click More and Delete.

Hi, if you are using the document module (which enables the "attachments" button on form views) you only have to click the X on the right of the attachment name in the attachment list.

If you are not using that module, and you have attachments on your messages, there is no easy way to do it since messages are not meant to be edited or deleted to preserve data over time.

Pura vida