Try using DBCC SHRINKFILE to shrink the transaction log file.
DBCC SHRINKFILE(
logical name of the transaction log,
new size)
You can check the BOL for more info on DBCC SHRINKFILE.
You will not be able to shrink the file past an open transaction. You can use DBCC OPENTRAN() to determine if there's an open transaction in the database.
-Mike