Stylebot lets me just write css, or can grab an element off the page and they give you a GUI to edit the most common style properties. The ability to just write CSS as simple as .stupid-ad { display: none; } is what I was after.

To anyone who wishes to do this, you can add more sites by listing them, comma-separated, except for the last entry, like so:

a[href^="siteone.com"]::after, a[href^="sitetwo.com"]::after, a[href^="sitethree.com"]::after { content: "[generic warning because multiple sites]" }