Hi Vargsa,
According to my research, there seems to be no such a registry key or GPO to disable this option. Besides, instead of Encrypt button under Options tab, you can also find another button via File > Encrypt in New Email window.

Besides, to use Encrypt with S/MIME, users have to configure their certificate in Outlook. Otherwise, they cannot use this feature even they click on it.
As a workaround, you can also inform all users to avoid using wrong Encrypt button.
If you have more concerns on this feature, it is encouraged to visit the Outlook user voice forum:
https://outlook.uservoice.com/ to vote for a similar idea or post your own.
Thank you for your understanding and support.
Regards,
Perry
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