One solution not mentioned is the use of a two drum winch design with one cable from one drum pulls up while the other cable pulls down .
A spring or latex tubing attached to your lift with continuous upward force properly adjusted to your lifting weight will work fine to control motor/ gearbox back-drive. It really makes your winch work to pull the lifting arm down first
and less force to pull weight up.
http://info.vulcanspring.com/first-robotics-competition
Check out voucher from Vulcan Spring . You could add this quickly to your bots
lift to stop the back-drive with out changing your design as long as your winch
is not relying on a gravity return lift arm . Are you using a single cable drum ? Photo of your winch might be useful for us to help you solve the problem.
http://www.chiefdelphi.com/media/photos/41279?