Spam through Webhooks

Greetings,

Something annoying happened today. I have webhooks in place for every change to get delivered to a Discord server. Suddenly some insulting spam comments kept showing up. When I follow the link to the alleged comment, it doesn’t exist (not shown on the site). I also can’t block the username because it 404s both on the regular user URL and in the blocking search box.

How to mitigate this? Is this a known loop-hole? Why can a nonexistent user keep generating spam comments?

Thanks.