tr.athing:has(td a[href="substack.com"]), tr.athing:has(td a[href="substack.com"]) + tr, tr.athing:has(td a[href*="substack.com"]) + tr + tr.spacer {display:none !important;}

The above example CSS rules hide all submitted posts that link to substack.com, including substack subdomains (and the immediately following "spacer" TR)

Note: Be sure that your browser supports the :has() CSS pseudo-class.

[0] https://addons.mozilla.org/en-US/firefox/addon/stylish/