Try this:
SELECT New_accountid AS AccountId
FROM BingMapsPlatform_MSCRM.dbo.FilteredAccount WITH (NOLOCK)
WHERE new_islicensed = '1'
AND CONVERT(NVARCHAR(100),New_AccountId) NOT IN (SELECT DISTINCT CONVERT(NVARCHAR(100),New_AccountId) AS New_AccountId
FROM BingMapsPlatform_MSCRM.dbo.New_keydenylistExtensionBase WITH (NOLOCK)
WHERE New_AccountId IS NOT NULL)