Thanks for the report however that's working as intended. Not all entities can be disabled through script - cargo wagons, locomotives, and such also can't be disabled.
If you want to get ahold of me I'm almost always on Discord.
It was possible in 0.15 and previous. 0.16 made it not possible as a result of the transport belt optimization. The belts themselves aren't updated at all anymore and all of the logic was moved into a central manager class that handles moving the items for entire merged sections of belts.
If you want to get ahold of me I'm almost always on Discord.