There is no built-in command to achieve this.
You can turn off all e-mail alerts in the TFS Admin Console by unchecking the "Enable Email Alerts" checkbox.
Alternatively you could also remove disable SMTP settings via command line:
%ProgramFiles%\TFSConfig ConfigureMail /Enabled:False|True
(Special thanks to Jim Lamb, Microsoft, for making me aware of the command line, and Martin Kulov and Tiago Pascoal for the idea to just disable the SMTP settings.)
- 03/28/2011: Changed the command line parameters from using the SMTP settings (/FromEmailAddress:EmailAddress /SmtpHost:SMTPHostName) to just use "/Enabled:True|False" (Thanks to Rich Hundhausen for making me aware, Thanks to Jim again for providing the undocumented parameter).