How big a problem is this for you? How many times has a prospect taken your ideas for free (never to come back) compared to to the number of times this type of pitch has been profitable?
If it's just a small percentage, you could consider it a cost of doing business. If on the other hand it's becoming a regular occurrence and it's really denting your profit, I can only suggest you start charging for your time and work on a good way of 'selling' the reasoning behind this to all your prospects.
I mean let's face it, most of what you are being hired for is the 'idea' rather than the donkey work involved in the execution, so it seems unwise to give these guys those ideas for free. If you sell solutions to problems, and you end up solving those problems as part of your free sales pitch, it's inevitable that a lot of people will take the free advice and run.
I don't personally know of any consultant who 'gives the game away' as part of his sales pitch. I'm not saying this can't work, I'm sure some prospects appreciate it, but it doesn't sit well with me personally.
You could give tasters, general advice, point to previous work in a similar field, testimonials etc, but if they want specifics, I'd be asking for some money upfront.
We've been slowly climbing the ladder and have been getting opportunities to pitch to bigger and better clients.
TBH in my experience, bigger clients are least likely to expect your time for free in the first place. And for me, 'better' clients are the ones who truly value your time and expertise and are of course, willing to pay for it. One other solution to this problem is to revisit your initial qualification processes and work out a better way to 'weed out' the freeloaders and time wasters and instead concentrate on the truly 'better' prospects.