Here's a trick I use to reduce unwanted break-in-twos at train shows and the like. All you need is a black rubber hairband, as can be bought at Walleyworld. A lifetime supply costs a dollar or so:
1) Slip the rubber band over one of the couplers.
2) Twist band, slip over other coupler, twist again to make sure it's tight:
3) Them little indefinites ain't going ANYWHERE, even if a knuckle opens!
You can also wrap one of the bands around an operating coupler to keep the latch from moving.
Hope this helps!