Yeah absolutely, you’ve got to give it some teeth, perhaps if they ignore it silence them for a day and ramp up actions from there.
But to answer the general question, there’s currently not a way to apply user or group based restrictions on a topic level. You could create a group, add them to it, and then prevent the group from replying to topics in the whole category… but understandable if that’s too broad.
There’s a related request here: Restrict Topic to Specific Users — but note it would be a fairly complex feature to implement, so it’s unlikely something we’ll be able to prioritize unless we get many requests for it.