General

You need to be an Administrator in order to delete users.

MediaWiki don't support the deletion of user accounts out of the box, but I've added a module called "UserMerge" that will do the trick.

Steps

Open up UserMerge in a fresh tab.
Open up the list of users in another tab, sort by creation date, and go to "Last" to get the list of most recent user registrations.
Look for suspiciously looking user names, e.g. "Faac12Fe".
Open such users pages in a fresh tab in your favorite browser
Check that there isn't a presentation on that page describing who this really is
Check this user's contributions (link in toolbox to the left saying "Contributions")
If no description or contribution is present, copy the user name and click "Block user" (in toolbox to the left)
On the block user page, choose "Infinite" as expiry for this user
Check "Other reason" and enter a reason such as "Bot registration" in the textfield
Keep default checkbox selections and press "Block user"
Switch to the UserMerge tab and paste in the user name as "Old user"
Enter "Anonymous" as new user to merge with
Check "Delete old user"
Press "Merge user"
Repeat steps with next user in the user list page