Do NOT do this without a recent backup and only if you are really sure you want to do this, otherwise you could lose data:
USE DbName
GO
EXEC sp_dboption N'DbName', N'trunc. log', N'true'
GO
DBCC UPDATEUSAGE (0)
GO
DBCC SHRINKFILE (DbName_Log, 100, TRUNCATEONLY)
from http://www.sql-server-performance.com/forum/topic.asp?TOPIC_ID=6818