Can the spin wheel pop up only when a shopper is about to leave?

Yes. Edit your campaign in Plugins → Spin Wheel and change Trigger:

• On page load: shows straight away.
• After a delay: shows after the Trigger delay (seconds) you set.
• On exit intent: shows when the shopper looks like they are leaving the page.

You can also limit where it appears with Show on: All pages, All product pages or Specific products (pick them under Target products).

Only one wheel shows at a time. If you have several active campaigns, the one with the lowest Display order wins. Click Update campaign to save.