BACKUP LOG <DatabaseName> WITH TRUNCATE_ONLY
This was for 2005. Sorry, I did not see the tags - its better if this information was in the subject or in the body of the question.
Anyway, I googled this for you:
From: Nick Kavadias
You can send the file to the bit bucket with the regulat BACKUP LOG command:
What does is discard your transaction log, so if your database is in bulk or full logging, you now have a broken chain of transaction log backups (which you should be taking at often intervals). So please, make sure if you run this command take a full or differential database backup afterwards!
If your in an environment where you dont need to do log backups then consider switching to a simple recovery model.