Remove Decimal

It would be easier to give you the right answer if we had an example to work with, but the truncate function of the round function may give you what you need:

declare @YourDecimal decimal(8,4)
set @YourDecimal = 1.1234

select cast(round(@YourDecimal,3,1) as decimal(8,3))

The third argument of the ROUND function allows you to specify that the rounding should be done by truncation rather than 'normal' rounding, so you can use this to truncate the value to a decimal with 3 decimal places, and then re-cast that, otherwise the ROUND function returns the same scale as the value passed in.

The method depends on the source data type. Some will round others will truncate. You seems to be looking for a method that truncates.
CAST and CONVERT BOL - http://msdn.microsoft.com/en-us/library/ms187928

-- This is an example where conversion to an INT from a Numeric truncates the values
DECLARE @Values TABLE ( Val DECIMAL(8, 4) )

INSERT  INTO @Values
       ( Val
       )
       SELECT TOP 2000
          ( ABS(CHECKSUM(NEWID())) % 5000.0 + 1 )
          + ( ABS(CHECKSUM(NEWID())) % 2000.0 + 1 ) * .0003 AS [value]
       FROM  [sys].[syscolumns] AS s
          CROSS JOIN [sys].[syscolumns] AS s2

SELECT  val ,
       CONVERT(INT, val) AS [converted]
FROM    @Values AS v