The short answer is that you can’t. If you’re very concerned, you can limit your pitch to the rough outline of the idea without giving away key information.
The longer answer is that this is not something worth worrying about. Unless you are confident your idea is a ‘key-in-hand’, easy-to-implement innovation that hasn’t been thought of yet, the advantages gained from getting broad-based feedback and a strong team motivated by collective ownership far outweigh the remote risks of someone stealing and executing on your idea.
The truth is that over 90% of ideas pitched have already been pitched - probably many times - in the past. This doesn’t imply that the idea isn’t a good one, but rather that what truly matters is how well you and your team execute the idea. “One can steal ideas, but no one can steal execution or passion”