fabiomcaldas Well, I'd politely explain that unfortunately I can't lower my rates. After all we've already negotiated them before we started working, so trying to lower them would be a breach of a contract.
My current clients never really question my rates and accept them without hesitation. However 3-4 years ago when I was working with the agencies there were a few situations when they tried to put a price pressure on their translators (in the form of impersonal mass email).
In this situation the only thing to say is: