You could use Events System code to prevent the members of that group from creating keywords in the lower publication. However, I'd consider this a workaround/hack and agree with Dominic that this should be handled using permissions.
Another option that you may want to consider is using a new group specifically for this purpose and using the scope to ...
While, as you say, localisation should be used carefully, localising the category in this case is probably the right thing to do.
If you have a scenario in which localising a category would cause you problems, that's probably best dealt with as a separate question.