With the recent deployment of issue #3716, marking artist URL links as dead is now possible. Take a look at artist #48811 as an example I just did.
To mark an URL as inactive, in the edit box all you have to do is precede the link with a hypen "-".
In the HTML view, the links show up with preceding hyphens. In the JSON/XML views, the URLs have a new field called is_active
to denote this.