Delete every users from a group
In general you shouldn't delete a group because some files may belong to that group but it may be sufficient for your specific sudo usecase.
A more general usecase is to use
gpasswd (1) at it allows you to set the members of a specific group, so it should be enough to run:
gpasswd sudo -M ''
To only run this if the group sudo exists you can combine it with a getent call, i.e.:
getent group sudo && gpasswd sudo -M ''
There's another way using command
/usr/sbin/groupmems ("administer members of a user's primary group") on systems that have this command:
groupmems --purge --group <group_to_clean>
or using short options:
groupmems -p -g <group_to_clean>
All members will be removed from the group.
Tested in CentOS and Fedora.