Head to your wiki and enter "MediaWiki:User-identity-box-group-blocked" in the URL bar (not in the Wikia search bar. It won't find those pages until they've been edited at least once) behind the "/wiki/" component.
There, it'll display "Blocked" in a box. Hit Edit, change it to whatever you like as long as it's appropriate and hit Publish.
On the topic of editing existing user-tags, you can also enter the following instead of blocked: