Is it possible in CPanel to restrict the assignment of a channel entry to a single leaf node category? My categories are hierarchical and only a single, bottom-most category is valid per channel entry.
You can try Lonely Cat
Which will set a single category.
Possibly needs @Cheyney's answer too (Not auto-assign category parents)
"Assign", yes. "Restrict", no. Unless you find a plugin.
You can ensure that an entry is NOT assigned to categories above the node chosen by setting "Auto-Assign Category Parents" to "No".