is there any way to block emails in Apple Mail by Subject line on all my devices?

You can create a rule online at iCloud.com that will move any email with certain words or phrases in the Subject field to the Trash bin. It won't block them but will send them directly to the Trash bin. You'll have to delete them.

You'll have to go to iCloud.com and log on with your Apple ID and password.

Randall_2023 wrote:

If you go to your email provider using the web access, you might be able to accomplish it there. Then all your devices will receive email based on the rules you create on the providers server.

If you go to your email provider using the web access, you might be able to accomplish it there. Then all your devices will receive email based on the rules you create on the providers server.