This is probably industry specific, but I will say this:
As a developer, the best clients for me are ones that will work in a partnership on their projects. I can't build a successful application for a customer if I don't have their regular interaction and guidance. This is probably also true for creative endeavors such as logo and website design.
Poor prospects (those who are tire kickers or other freelancers looking for price info) won't spend the time and effort answering questions about their project.
A good way that I have found for weeding out some of these non-real requests are to ask a series of questions of the prospect to see their response. Optimally this would be a somewhat generic questionnaire that you could send to all prospects as a first step in your process. My questionnaire might ask:
- Describe the goal of your project. What are you trying to achieve?
- How will be measure the success of the project?
- How many users do you anticipate having?
- Do you need to support international users?
- What kind of testing resources will be available during development?
It shouldn't be completely overwhelming to the prospect, but the reality is that asking questions like this can be an eye opener to someone who doesn't really understand the work involved in a project like this. I'd prefer to work with someone who has realistic expectations from the beginning.
And for tire kickers and competitive-spies, they are unlikely to take the time to respond. Which is great, because now you don't have to pursue this any further.
Related: If this is a big problem for you, you may need to re-examine your marketing to determine why you are getting so many of these types of requests. You may need to tighten up the language or find better marketing avenues.