Brainstorm a product idea for which recruiters are the core market segment. Every time a recruiter calls, treat the call as a free half hour customer development interview or usability study. As you continue to refine the idea, build some mocks/prototypes that you can have the recruiter walk through while you're on the phone.

Two possible outcomes:

- The recruiters quickly learn you have no intention of talking about job opportunities, and stop calling.

- You develop a very well validated product that you can effectively sell to recruiters, a space that is red hot right now.