The permission Access Marketing Activities - Activate Smart Campaigns is very effective to prevent unauthorised users sending emails.
Granting "Access Lead Database" exposes, implicitly, the ability to select multiple leads and send emails through Lead Actions - Marketing - Send Email.
Bypassing Activate Smart campaigns can cause more damage than sending campaigns accidentally since it is possible to select larger number of leads than a specific campaign would allow.
The "Idea" is a new option under Access Lead Database in order to control who can send emails, ideally warning admin users when Access Marketing Activities - Activate Smart Campaigns is not granted to that role.
It is important to retain Design Studio - Email Actions - Send Test. It could have a setting in Admin in order to limit the number of addresses entered in the "Send to" input field A regular expression would help to count the addresses entered.