Dealing with GST as a hub

Hello all I have just started a hub on OFN we currently have 8 producers but expecting more. The majority of products are GST free with a few exceptions. The hub is registered for GST but some producers are not:

  1. We have one producer who currently has all her products that require GST but she is not registered for GST.
  2. One producer is registered for GST but has products that both requires GST and some exempt.

I realise I can ask the producers to select the items that require GST in the Products>Product Details>Tax Category. However I then have to rely on them to do this (ok with current producers that I know and trust but what about new producers I dont know?). How can I control this as the hub? Or what is the best way to manage the 2 scenarios above?