So "Go to superspammysite.com for horny pictures" is still an allowed username, right? Or of you skip spaces "superspammysite.com" may be enough that some spammer wants to abuse it.
I think you can't restrict usernames enough to not allow any spam. The only right way is to really just have the email and nothing else in the first confirmation mail.
Then you have "Hello superspammysite.com, welcome to our website." Not enough space for some kind of scam-text like in TFA. And considering that this has not happened in the last 27 years, it does not actually seem that worthwhile for spammers.
I think you can't restrict usernames enough to not allow any spam. The only right way is to really just have the email and nothing else in the first confirmation mail.