Go to the Admin section of your board, then, click on "Inactive Members for at least XX days". Click on the 'View' button. The default for XX, is 30 days I believe. Click on it, and it will bring you to a screen, that shows people who have been inactive longer than XX(30) days.
Once you get in that place, it will show the person's shown public name (followed in quotes by their username), their Post Count represented by a number, and an unchecked checkbox. Check anyboxes on members you want to delete, and scroll to the bottom and click on 'Delete Selected Members'
Easy as that
