There's no way to disable audit trails, but if you don't want to use them you can just not show the Audit Trails tab by removing it from System Options -> Main Tabs. In the upcoming 8.1 users will be able to just close the tab and not have it show.
I would recommend against deleting any of the audit trails data from the database. If you are not showing the Audit Trails tab, the perfomance implications of shrinking that table are minimal, the table is only accessed when saving or deleting an item.
Dan Suceava
http://www.axosoft.com