I'm not sure that I understand what you mean.
If the space bar makes the Selector or Proximity Selector component interact with the NPC's Dialogue System Trigger, disable the Selector/Proximity Selector during conversations. (See 07:00 of the Interaction Tutorial.)
If you disable the Selector or Proximity Selector, it will hide the selector UI at the top ("A // (spacebar to interact)").
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Regarding "Another conversation is active": If you want one conversation to interrupt another conversation, tick Replace:
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