Используем что-то типа такого:
USE [base]
GO
-- Truncate the log by changing the database recovery model to SIMPLE.
ALTER DATABASE [base]
SET RECOVERY SIMPLE;
GO
-- Shrink the truncated log file to 1 MB.
DBCC SHRINKFILE (N'db_тутимялогфайла_log', 1);
GO
-- Reset the database recovery model.
ALTER DATABASE [base]
SET RECOVERY FULL;
GO