If you want to prevent someone from stealing your idea you need to have them sign a Non Disclosure Agreement (NDA). I insist on NDA's when I talk about products with development companies. These organizations usually transform into product companies after starting out as development companies. The last thing I want is for that idea they execute upon to have been one I'm in discussions with them about.

On the other hand, if it's just a friendly chat with an advisor I'm much less focused on the NDA. It takes a lot to build a company and NDA's aren't everything.

If you want to stop someone from stealing your idea just do nothing, take no risks, and find a cubicle at a large company to hide in. If you want to do something big you always have to put your ideas out there for others to steal/imitate.