You will find some code similar to these in your skins:
< dnn:USER runat="server" id="dnnUSER" / >
< dnn:LOGIN runat="server" id="dnnLOGIN" / >
The first on displays Register/DisplayName and the second displays Login/Logout. You can remove either or both of these from a skin.
If you are interested in some possibilities for conditionally including these or other items in your skin or pages, take a look at
this thread in the Snapsis.com.
I've use techniques found in that thread on a site that I run. The USER and LOGIN controls aren't displayed when the site is viewed by an unauthenticated visitor. Instead, they see the Login module (visibility controlled by Security Role). When a user logs in, the Login module disappears and the USER and LOGIN links, as well as some other stuff) is shown in the upper right. Turning USER and LOGIN on and off is accomplished with conditional code in the skin.