Group administration screen allows you to Remove groups. When you select a group that has permissions associated with it (but in our case, there were no users who were members of this group) and click Remove you are given a list of spaces (but not personal spaces) and space permissions associated with the group. Then at the bottom of that list is a Confirm button to agree that you will lose all these permissions, etc. Click Confirm and you get an error message with the following trace:
If you then go through and manually find all the spaces and remove the group from the permissions list for each space then you can finally remove the group. Big problem if the group has access to personal spaces as the only way you can find that out is by querying the database - the personal space permissions do not show up on the aforementioned permissions listing.
You should be able to actually remove a group and all its permissions as long as there are no members of the group. Group removal should not be restricted to cases where there are also no permissions associated with the group, especially because it is impossible to see all the group's permissions (i.e. personal spaces) without querying the database manually.