try this:
The last username information is actually stored locally on the client side, not on the 2008 server. This information is held in the
\MyDouments\Default.rdp file and in the registry at
HKEY_CURRENT_USER\Software\Microsoft\Terminal Server Client\Default. You will find a bunch of MRU keys that you should delete. You should also delete the keys in HKEY_CURRENT_USER\Software\Microsoft\Terminal Server Client\Servers.
Good news is the display of the last username can also be controlled on the server side.
Go to Start > Administrative Tools > Remote Desktop Services > Remote Desktop Session Host Configuration.
In the window that appears, right-click on the item in the "Connections window" and click properties. In the "Log on settings" tab, check the middle option, "Always use the following logon information" an leave every field blank. Click, apply, Ok. Logout and log back in. The last user name should be gone form the RDP screen.
You really have two choices in how to handle this:
1) Delete RDP file and registry items on the client every time it is used for RDP
OR
2) make the changes on the server to ignore the client's supplied logon info.
Good Luck!
By, Balou1