The frustrating thing about this problem (since the RSS subscribe menu item appears by default) is that the option to disable it is only visible when using the “Thesis Nav Menu” – which is now disabled by default, in favor of the WP nav menu system.
If you go to Thesis Site Options -> Navigation Menu -> Select Menu Type, you can switch menu systems, if only just long enough to see the “Feed Link” option appear below – now you can disable it!
Make sure you save changes, then open Select Menu Type again so you can switch back to the WP nav menu system.
The subscribe button is now gone!