To help with additional details, here is how it should all look so you can say exactly which portion isn't correct. You have a login for the server, and a user for the database for the account you're having trouble with. On the server and database levels, you'll see a public role (server role and database role). The properties of the public server role won't show users, but the properties of the login will show the public database role for all of the databases (it will be checked, and you can't uncheck it). The properties of the public database role will not show any members either, and the properties of the database user will also not show the public role. Because of all of this, I don't believe you removed the user from the public role.
If you want to try to delete and recreate the user, you can first try to go into the database and just delete the database user. This will leave the login, which you can go into the properties of, and, under User Mapping, there should be no database checked. You should be able to check the database now (because the database user has been deleted), and choose whatever roles you need. If this doesn't work, you can also delete the login along with the database user to try and clear everything out. For one last check on the user, you can run this SQL:
use database_name
exec sp_change_users_login 'Report'
This will show you any orphaned users, which could mean there is still an issue with your user. This can usually be fixed with:
use database_name
exec sp_change_users_login 'Auto_Fix', 'username'
To try and wrap this up before it gets any longer, there could be something else altogether causing your connection issue for this user. If none of this works, can you post up the error message you get when logging in?
Was there a SQL Express installation on there at one time? The thing confusing me is that SQL Server 2008 R2 Express has a 10GB limit so I have no idea where the 4GB is coming from. Maybe the upgrade process from 2005 Express to 2008 R2 isn't working properly and doesn't change the database size limit.
My best advice would be a complete reinstall of the SQL software. The only way I can imagine that a 4GB limit would be in place is if SQL Server 2005 Express was installed first.
Best Answer
The disabled user shouldn't cause issues but if it's removed from AD sp_helpdb will start throwing an error. Changing the owner takes some testing to make sure you don't break any applications or processes using the database.