Joomla and phpBB seem to disagree on what is a valid username. phpBB allows pretty much anything, is there any way I can make Joomla CB allow this as well? Some people have special characters like ! or +. Existing users can login fine, but if they want a special username I've found that they need to register through phpBB and then I sync them in the Admin panel.

Any idea where I can stop the restriction in Joomla?

Not sure you can get around the restriction in Joomla. We have a reported bug on this topic and the planned fix is to give the admin the ability to select SKIP or RENAME when importing... this would either not import the user of would rename then removing all illegal characters.

Sometimes the usernames with special characters causes errors when importing into Joomla... this is what the original bug report was centered around...

Greg