Can I disable a battery powered device by reversing half of its batteries?
If the batteries are not perfectly balanced they would still have some net voltage. And that is assuming that the batteries are connected in series.

If the batteries are connected in parallel, flipping one of them the other way around will basically create a dead short between them. At best, that is going to drain them.

In either case, storing batteries in a device is not a great idea. If the device ends up being stored for a longer period than you anticipated, and the batteries start leaking, the internals of the device is hardly an ideal place for them to spill their fluids.

Retrofitting an off switch would be better, although that still doesn't solve the issue of long time storage and leaking.