My advice for a mechanical solution would be to include a worm gear to prevent being backdriven.
Other options would be to leave the calibration off-true just enough as to equal the force down so that at 'rest' the motors would turn to fight being backdriven. As a general warning, the first method is a tried and true method. The second is me thinking on my feet (ok more-so thinking on my tush).

Michael Greenley