It couldn’t be easier to make sure there are no external contacts in your Threema Work group chat: only add contacts with blue dots. https://threema.ch/faq/levels_expl
🔴⚪⚪ external contact
🟠🟠⚪ external address book contact
🔵🔵🔵 verified internal company contact
Of course, you could also restrict communication to internal contacts (https://threema.ch/work/support/closedgroup) or use Threema OnPrem to begin with: https://threema.ch/onprem