How to block 'iCloud storage is full' scam emails?

You can block emails from a specific sender, but the problem is that with any spam, they spoof the sending address to make blocking ineffective. If you ever click a link in any of these emails, then it alerts the spammers that you may be susceptible to believe what they say and you will continue to get more along with additional spam emails.

Flag or block emails in Mail on iPhone - Apple Support

If you have a Mac available, a rule can block those.

iCloud mail filtering is limited, but might be enough:

Set up rules to filter email in Mail on iCloud.com - Apple Support

Otherwise, mark them as junk.