This minprice is obtained by an SQL function. In your db, go to
Programmability > Functions > Scalar-valued Functions
fnKartrisProduct_GetMinPriceWithCG
From looking at that, i think you need to cut this bit out:
DECLARE @QD_MinPrice as real;
SELECT @QD_MinPrice = Min(QD_Price)
FROM dbo.tblKartrisQuantityDiscounts INNER JOIN tblKartrisVersions
ON tblKartrisQuantityDiscounts.QD_VersionID = tblKartrisVersions.V_ID
WHERE tblKartrisVersions.V_Live = 1 AND tblKartrisVersions.V_ProductID = @V_ProductID
AND (tblKartrisVersions.V_CustomerGroupID IS NULL OR tblKartrisVersions.V_CustomerGroupID = @CG_ID);
IF @QD_MinPrice <> 0 AND @QD_MinPrice IS NOT NULL AND @QD_MinPrice < @Result
BEGIN
SET @Result = @QD_MinPrice
END
But you might need to tinker with it to get it to work. Note that this will still pull out customer group pricing for an item if such a value is set.