Technically, it's not your data. Your mother voluntarily signed up for a Skymiles account (using your help). You may disagree, but I think it's a positive that Delta won't delete an account solely based on someone saying the account holder is deceased.

I'd probably skip emails or sending messages online. Those are easy to get misinterpreted. Call. If the person doesn't understand your request, you can discuss it immediately. You may not get the account deleted (without providing the death certificate), but they should be able to address the promotional emails.

From the Cookies, Privacy & Security page of the Delta website:
"How Do I Opt Out of Being Contacted by Delta for Promotional Purposes?
• You may call our Customer Care Offices at +1-404-773-0305 if you have any questions about changing your contact preference or require assistance in removing your name from our subscription lists."
Source: https://www.delta.com/content/www/en...-security.html