In Vuetify, how to prevent a checked v-checkbox from being deselected?

I have a checkbox:

<v-checkbox
     v-for="company in allCompanies" :key="`company_${company.id}`"
     :label="company.name"
     :value="company.id"
     v-model="selectedCompanies"
/>

How could I prevent a selected box from being deselected without having to resort to using radio buttons?

Or how to, using Javascript, check whether a specific checkbox has been deselected?