Instead of preventing people from subscribing it is possible to put them in a Pending queue.
Pending group members can to be reviewed and approved by a user with the site wide role "Administrator" -or- Group Manager and/or by a member with the group role of either "administrator memberog" or "content editor".
- This is done from the Group tab (note: the tab being refered to is the light grey admin tabs and not the main main tabs!) and then by clicking People then filtering by State "Pending".
- You may then approve them one at a time or all at once via Operations > "Modify membership status".