You have two options when iTunes decides to do this:
1. Rename the artist to something random, it doesn’t really matter. Once that’s processed,
go to the new artist you created and then retag it back to the proper artist.
2. Remove the files from iTunes entirely and then re-add them. They’ll be fixed when reimported.
The former generally works for me, but I have to sometimes do the latter too.
Also, I suggest using a better tagging program. iTunes has been an utter mess since around the 10.0 update. I don’t know what OS you’re on, but there’s mp3tag for Windows and Yate for OSX. mp3tag is free, but if you need Yate, send me a PM.