Disable automatic login in pgAdmin
The root nodes in the "Object browser" are actually connections, not servers. And a connection requires a user name. There is a bit of mis-labelling floating around, I have raised this issue on the developer mailing list before.

However, you can just edit the properties of a connection before you open it. If you don't want to store your password for automatic login, don't check the box at login time. Once you change the username in the properties you have to enter a new password automatically.

You can also save multiple connections to the same server with different usernames.