How can I disable Rickroll in Visual Studio Code even if it is not installed?

As you can read in the Details section of the extension, it does modify the desktop.main.css file, so in order to remove the rickroll you have to run the command Rickroll remove Rick from the command palette. Done that, you should be able to uninstall the extension and reload the window.

Edit: If the problem persists be sure to have fully deleted, in the VSCode_Installation_Folder/Microsoft VS Code/resources/app/out/vs/workbench/workbench.desktop.main.css file, the code between /*ext-vscode-rickroll-start*/ and /*ext-vscode-rickroll-end*/; then, after making sure you've uninstalled the extension, reload the window by the command Developer: Reload Window

If you only uninstall the extension the CSS is already modified, so the background will stay there.

In order to access the command Rickroll remove Rick the extension must be installed but this command does not deactivate the watcher that triggers the Rickroll, run the command and immediately after uninstall. If you only run the command you will eventually get Rickrolled again.