I searched discourse meta to block rickrolls. But I found what? A user with the name of that username. We mods blocked the first main line of the song, but users could post different newly generated links. How can you block them? We mods failed to do it.
AI-generated summary
The discussion revolves around detecting and blocking unwanted content on a forum, specifically rickrolling. S_B_2_Go shared their experience with trying to block rickrolls by adding URLs to the “Watched words” list, but users found ways to bypass the block. bartv suggested adding the video URL to the “Require approval” list, but acknowledged that users could use link shorteners to evade the block. JammyDodger emphasized the importance of addressing unwanted behavior at the person-level, through moderation and community guidelines.
S_B_2_Go expressed frustration with not being able to automate the process and having limited editing capabilities. maiki pointed out that any filter can be bypassed with creative workarounds, and echoed JammyDodger’s advice to address the issue through community guidelines and moderation.
Ultimately, maiki suggested that the moderators should discuss the issue with the community’s administrators to find a solution that works for the community. By working together, they can assess the need for technical solutions and develop a plan to address the issue.
You can add the URL of the video to the Admin > Customize > Watched words > Require approval list. Users will probably start using link shorteners to get around that, so you’ll have to add those URLs too.
I think all the auto-tools are really useful, but they do have their limits. Sometimes the best way to deal with unwanted behaviour is at the person-level, and tackle it as a moderation issue.
If it’s something that you feel is not acceptable for your forum, then explain why it’s undesired, write it into the forum guidelines, encourage users to flag, and give warnings to those who continue to ignore the rules.
There isn’t a plugin to automate this?
I can’t edit that. But I can edit topics created by me and others.
But they could post gifs of Rick, which isn’t funny.
@S_B_2_Go, let’s put aside the issue you are addressing for the moment.
Any filter you set up against me, I can sit here and think of a way to get around it. If I’m the rickrolling type, I’ll get my video links and lyrics in somehow. Maybe I’ll take a photo of the lyrics, or I’ll share an ASCII version of the video. It will be weird, it will get through, and I’ve got all the time to do this.
If there were, then we’d be pitting humans thinking up silly ways to rickroll folks and the plugin would take so many evasive actions eventually it would interfere with the operation of the site; meaning now it’s affecting people more than just annoying topics being published.
Hence this sound advice:
Okay, turning back to your specific case, I noticed…
This sounds like you and the mods have an opportunity to discuss this with the admins in your community, and address it together! 
If you are having trouble with a few users, talking to your team will provide more perspective, as well as more tools to use that already exist. And if it is many users, then maybe they need a place to be silly, and the admins need to know about it. In any case, you can work with your community staff to assess what, if any, technical solutions you need to apply. 