Since LS is still lacking a answer relevance, you would need to create a Question validation equation.
Other workarounds (which might work):
1. Create a multiple short text question with default answers filled with an equation that fills only when NOT NONE. Hide this question via CSS and use that question as array filter on your radio list question. That should filter all NONE entries.
2. Use a Multiplechoice question with subquestion relevance. Limit answer to max. 1