Hi,
You control who can post on your blog through the setting at:
Options > Users > Change permissions
By default those in the "Anyone" group can post. Most administrators turn off the "Reply" permission for those in this group but make sure it is on for "Registered" users. "Subscribers" I.e. those who have opted to receive posts by email, must be registered as they will have supplied and verified their email address.
If you turn off the Reply permission for Registered users that should stop even comments posted by email from being accepted on your blog.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.