Hi all,
Just a note about deleting users. I have a wiki that was spammed with about 100 "junk" usernames (made up of random characters), some of which were then used to vandalize the content. After a little fruitless effort with the DeleteUser extension (http://www.mediawiki.org/wiki/Extension:Deleteuser) I figured out how to delete them all in MySQL.
Wasn't difficult at all - just copied the userlist into a text file, weeded out the real usernames (which were in the minority), inserted more text into each line to make it an SQL command, and executed it all at once.
This was a situation where I really needed to delete - the junk had rendered the userlist useless, and none of the spammers had contributed anything that I wanted to keep. Haven't seen any ill effects. And of course now I've locked down registration to keep the spammers out.