I assume you are using the WordPress.com Stats plugin.
On the dashboard, under the Plugins menu on the left, look for ‘WordPress.com Stats’, and there will be an option called ‘Count the page views of registered users who are logged in’, which you should deselect.
This discounts any user that has logged in. If you allow local accounts, and use them for users to leave comments, they’re gone from the stats too. Generally though, this is fine for most blogs, but if you want more control, you can Google Anayltics and if you use Joost de Valk’s Google Analytics for WordPress plugin to insert the tracking code on your site there’s an option under advanced settings where you can decide which type of user to ignore (Admin, editor, author, contributor or subscriber).
