is there a timeline for the next release?
forge.joomlapolis.com/versions/566
This would help to stop people registering with that kind of usernames.
I've provided 2 ways to fix the issue in my previous reply. That's the best I can offer at this time.
Perhaps the function for the username field might do the job?
The username field does not strip anything. Fallback usernames also don't use the field validation. If you are actually using the username field then yes you can adjust the validation to not allow spaces.
Even with release of CB 1.10 it's not going to fix existing installs. It will only help future usernames to always be valid. For existing users you need to change the usernames, have them change the usernames, or change Joomla to allow spaces again. We absolutely can not put anything in CB to automate changing an existing users username as that's just too dangerous.