I imaging possibility of using ::after to create a window, which completely blocks Derpibooru and says "pay X to Y to unlock this site"
"@Background Pony #0890":/meta/userscripts-customization/post/4354748#post_4354748
As byte says, using elem.style.prop = x instead of elem.style = "prop:x" bypasses issue.
Last time I tried Violentmonkey, it didn't work with Derpibooru *at all*. Maybe stuff changed, though.