how to block an email account on iCloud.com

Is this an account sending Mail to you? If so, you can't block it, but you can set up a rule to automatically delete incoming e-mails. See Sample below which is from Mail. To implement the rule at iCloud.com/Mail, click the gear at the bottom left and select Rules. Set it up on that screen. If that isn't what you are trying to do, more information would help.

For iOS see this article Identify and filter junk mail in iCloud - Apple Support

You can not block the mail on server end , although can set rules for spam mails .

This does not seem to apply in iOS 11 versions??

I can't find a way to create rules for IOS. You could still create the rule at iCloud.com/Mail.