The real culprits..
Were you hiring it out 'dry'?
If ever there is an incentive to save some gas when flying somebody elses aeroplane......... I don't think we need to draw you a picture.
Change it to 'wet hire' and you control the fuel. The net result is zero incentive to lean the thing.
Consider also an engine monitoring system. The EDM 700 is STC'd. You can dowload the historic data onto your computer and presumably use the 'offending' traces to support your invoice for engine repairs.
Why not include a clause in the contract 'must operate the aircraft and engines in accordance with m'f's recommendations.'
This, plus the above evidence of abuse would probably stand up in court.
Failing that, do as I do and don't let anybody else fly your machines unless you know them well.
Lease an aircraft to a school and you will hear the following phrase on CofA - 'Take out your wallet and repeat after me, help yourself'.
HP
HP