Since the "Team Foundation Service" hasn't an administrative console, it's not possible to use it to delete a hosted project.
So to delete a team project from one Team Foundation Service collection you have to use the "tfsdeleteproject.exe" command line tool:
tfsdeleteproject /collection:https://<your collection>.VisualStudio.com/DefaultCollection <"Project Name">
For further information, see: http://social.msdn.microsoft.com/Forums/en-US/TFService/thread/81997146-a64f-43fb-9952-57d71542cd11