Remove ability to add email attachments
I don't think either of those settings would help prevent attachments for mailings. The first one the help text says "Maximum number of files (documents, images, etc.) which can be attached to emails or activities. This setting applies to UI forms and limits the number of fields available on the form." Setting it to 0 does remove the attachments field from activities, but the setting's not used I guess for mailings.

The second setting won't help. It's for things like the inbound email processor and is just a sanity/denial-of-service prevention limit.

I think this would need to be a customization.