How to force selection of a specific answer in a dropdown field within an editable grid

When you say "force 'not used'", what exactly are you hoping the sequence of events the users sees will be?

It would be trivially easy, for example, to set that dropdown to "required" and add a validation such that the only valid answer is "not used" when the selection in the previous column is "never".

Why not just hide the second dropdown?

I typically implement this by first having the "How Often" dropdown perform an a!save() to update the neighboring "How Vital" column, setting it to Not Used when Never is selected, then have the How Vital column become disabled in the same scenario.  You could take this further and prevent "Not Used" from being selected with validation when any other selection of How Often is selected, or have the How Often selection simply clear the How Vital selection whenever it is toggled otherwise, etc.

Essentially, you can easily reference data points across the row when you need to check other values:

a!localVariables(
  local!howOften: {"Daily","Weekly","Monthly","Quarterly","Never"},
  local!howVital: {"Vital","Important","Useful","Not Useful","Not Used"},
  local!data: {
    a!forEach(
      items: 1+enumerate(4),
      expression: a!map(id: fv!item, software: concat("Antivirus ",fv!item), howOften: null, howVital: null)
    )
  },
  a!gridLayout(
    headerCells: {
      a!forEach(
        items: {"Software","How often do you use this?","How vital is this to your work?"},
        expression: a!gridLayoutHeaderCell(label: fv!item)
      )
    },
    rows: a!forEach(
      items: local!data,
      expression: a!gridRowLayout(
        id: fv!index,
        contents: {
          a!richTextDisplayField(
            value: a!richTextItem(
              text: fv!item.software
            )
          ),
          a!dropdownField(
            placeholder: "-- Select --",
            choiceLabels: local!howOften,
            choiceValues: local!howOften,
            value: fv!item.howOften,
            saveInto: {
              local!data[fv!index].howOften,
              if(
                local!data[fv!index].howOften="Never",
                a!save(local!data[fv!index].howVital,"Not Used"),
                {}
              )
            }
          ),
          a!dropdownField(
            placeholder: "-- Select --",
            choiceLabels: local!howVital,
            choiceValues: local!howVital,
            value: fv!item.howVital,
            saveInto: local!data[fv!index].howVital,
            disabled: local!data[fv!index].howOften="Never",
          )
        }
      )
    )
  )
)