I just gave you a script that does it. You put the db into Simple recovery model. Then, to be sure, take a CHECKPOINT (I left that part out originally). Then, if you put the db back into full or bulk_logged mode, take a full backup right away.
DECLARE @recovery_model_desc varchar(60)
SELECT @recovery_model_desc = recovery_model_desc
FROM sys.databases
WHERE name = 'dbname'
IF @recovery_model_desc <> 'SIMPLE'
ALTER DATABASE dbname SET RECOVERY SIMPLE;
CHECKPOINT
DBCC SHRINKFILE( dbname_LOG, 2);
IF @recovery_model_desc <> 'SIMPLE'
BEGIN
IF @recovery_model_desc = 'FULL'
ALTER DATABASE dbname SET RECOVERY FULL
ELSE
ALTER DATABASE dbname SET RECOVERY BULK_LOGGED
END /IF/