If you are downloading email from Comcast on to a local client - like Outlook, marking them as spam just makes them spam on your machine. Spam filters have to be updated, and usually are if you are dong you updates. to block them at Comcast, go to Comcast.net and logon with your email user credentials, go to your inbox and mark spam messages. doing this, they won't get downloaded to you.
I do agree with using Gmail as their spam filters are good and once marked as spam, you don't get another from the same address. This also requires you go to you google account online and mark as spam - I'm suprised I can't mark spam messages on my i-phone, unless I just don't know how to do it. Maybe I'll look it up.