Bonds... not legally enforceable, I'm afraid. We do get signatures on training agreements, but that's more an integrity test than anything else - had a fair share of AFAP interactions when pilots decided on half-opportunities over meeting an agreed obligation. But that's in the minority overall.
We do invest in our people where possible - in my time we've financed initial twin endorsements, initial instrument rating assessments and full instructor rating courses - and we have no flying school capability. We're trying the approach of providing development to achieve lower crew turnover, but you'll forgive me if, during interview / recruitment stage, I view candidates with a level of skepticism - as prevention is better than cure!!