How to shrink the TempDB database without stopping MS SQL
Here's how:
DBCC FREEPROCCACHE
GO
DBCC DROPCLEANBUFFERS
GO
DBCC FREESYSTEMCACHE ('ALL')
GO
DBCC FREESESSIONCACHE
GO
DBCC SHRINKFILE (TEMPDEV,1024)
GO
Taken from : http://calyansql.blogspot.ca/2012/04/shrink-tempdb-data-file-without-sql.html