In 2010, we used a Dewalt Drill Transmission to drive our kicker mechanism, and it almost impossible to back drive it. There is a
white paper on how to attach it to most of our motors. You can also buy a Dewalt transmission from Dewalt directly for somewhere around $40 (can't find the link). They work really well.
The BIGGEST warning I have about them is to work carefully with it and follow the instructions from the white paper. They are extremely complicated and very hard to reassemble if they become disassembled (such as dropping it on the ground like we did at championships. Fortunately, there was some from Dewalt standing nearby who reassembled it for us in 15 seconds.) We just tried to service it, it took us two days to figure out.