How do you disable Post-based Membergroups?
Go to Members > Membergroups > Edit Membergroups
Click on 'Modify' for post count based member groups, and change their 'Group Type' to something else.
From the default Member groups, you cannot change type of 'Newbie' but you can change the other four :)
Yes you can't remove the first Post-Count group.
The only way to remove it from peoples accounts it to assign them to primary membergroup and turn of the display in your theme options.
Membergroups (http://docs.simplemachines.org/index.php?board=59.0;sort=subject)
I just renamed "Newbie" to "Member" and deleted the other post count groups.