I'm using this in Stylish, a CSS manager browser extension:

  /* Substack modals, nags, etc. */

  [class*="frontend-components-SubscribePrompt"] {
      display: none;
  }

Saved as "Substack" and applied to any page/domain which uses Substack.

Otherwise, for UBO, these two should work:

  ###.frontend-components-SubscribePrompt-module__subscribeDialog--2_6UY.frontend-pencraft-FlexBox-module__flex-align-center--21WCV.frontend-pencraft-FlexBox-module__flex-direction-column--2sph3.frontend-pencraft-FlexBox-module__flex--fK-9V
  ###.frontend-components-SubscribePrompt-module__background--2DQPj

I suspect Substack vary their element names (e.g., the "2_6UY" and "2DQPj" strings in the above list), so that substring matches as I've used in my CSS example should be more robust.

UBO uses the Easylist blocking syntax which ... I can't seem to find documented, so I'm guessing as to the specific patterns. Corrections/clues welcomed.