I've often wanted to force a single rdp/ts session per user, or sometimes just the opposite -- make each login open a new session regardless of user. While I was doing research about multiple remote desktop sessions on Vista, I finally tracked down the setting that controls the single remote desktop/terminal services session behavior. Here is the registry key:

HKLM\SYSTEM\CurrentControlSet\Control\Terminal Server\fSingleSessionPerUser

This is a DWORD setting. The possible values are as follows:

0x0

Allow multiple sessions per user

0x1

Force each user to a single session

You can also access this setting from the Terminal Services configuration snapin -- it's the Restrict each user to one session setting.