In the early days, we had a policy that we would not respond to RFP's. The time required vs. value does not make sense for startups (unless you've already been told you'll win and the RFP is just a formality).
That's also true for more-established companies, who often are just column fodder so that the procurement people can say that they've done their due diligence.
Basically tell them why its not worth your while to respond.
I believe that in a couple of cases, this may have caused the customer to rethink and then come back again later with an easier RFP.