How about, rare herbs found and prepared in the correct way deaden the druid's natural abilities. Or just keep him asleep for long periods of time and disoriented when he is awake.
Edit: C. Ross, I intentionally left this vague to be a suggestion for the OP to kinda run with. But in retrospect a more complete answer would be better.
The OP has tagged this as dnd 3.5. I don't have those books and the SRD has not real references to the barbarian's culture. In ADnD and 2e there were some references to barbarian's and their culture. Herbalism was mentioned as a non weapon proficiency for barbarians there and it would seem to fit in most cases. I would think it would be safe to assume that a nomadic clan of barbarians would have a shaman or healer with the herbal knowledge to create a concoction that would cause someone to sleep or at least be subdued. The rational is that as a healer sometimes patients need to sleep to heal faster.
I would also see no problem with the tribe having access to a rare plant that could be used to create a concoction that would prevent a druid from using their natural abilities. It could even be more general and affect a larger population such as shapeshifters in general.
One example in fiction comes to mind. In the Robert Jordan Wheel of Time books, there was a plant when used to brew tea would cause the drinker to loose their ability to connect to the "source" where they got their power.