Prueba asi:
Código sql:
Ver originalSELECT T0.CodCliente, T0.NomCliente
FROM TClientes T0
INNER JOIN TVendedores T2 ON T0.CodVendedor = T2.CodVendedor
WHERE T2.NomVendedor = 'Pablito'
AND NOT EXISTS(
SELECT 1
FROM TFacturas T1
WHERE T0.CodCliente = T1.CodCliente
AND T1.FechaFactura >= '20091001' AND T1.FechaFactura <= '20091010'
)