So many people are frustrated by this growing trend, not just among lowly spammers but big companies like eBay and Amazon too, that there is now an open-source Greasemonkey/Tampermonkey/Violentmonkey userscript to get rid of those irritating visual cockroaches:
Gmail Subject Line Emoji Roach Motel
How to Use It?
This script works in
Greasemonkey in Firefox
Tampermonkey in Chrome (and probably many other browsers)
...probably also Violentmonkey in various browsers
First, you have to download one of those monkey extensions in your web browser, by going to your browser's "Add-Ons" or "Extensions" screen and searching for the name of the extension and installing it. Scripts for those monkey extensions can give you all kinds of cool features.
Then, you load this script into that extension by simply clicking this link:
The script will only run when you visit mail.google.com and of course you can always turn it off in the menu for your monkey extension. When you first install the script, you will need to reload gmail or open a new gmail tab to see the effect.
What to Expect
This script does not modify your actual email, or delete or move any email messages. It simply prevents certain objectionable characters from being displayed while on the main gmail screen. If you turn off the script, all those characters come back.
This script should not slow down your gmail experience at all.
This script should only eliminate cockroaches in the subject line of emails in gmail's main screen (the screen where you see many different emails listed for one "mailbox"/label/folder), not the gmail screen showing one particular email with its subject and body.
This script should exterminate cockroaches in both the email title and the short snippet of the email body that appears on the same line.
There are two general types of emoji cockroaches that this script exterminates:
HTML-image-based cockroaches (including the horrible animated ones): this script eliminates image-based cockroaches before you have to see them.
Unicode cockroaches: The script exterminates these a few seconds after gmail loads, so you get the pleasure of seeing them disappear. There might be a technical way to eliminate this type from the start, but I haven't found it yet. When new mails with these cockroaches arrive, those cockroaches too will take a few seconds to be exterminated. Similarly, when you switch to a new folder, it may take a few seconds for cockroaches in messages of that folder to die.
The source code is available unobfuscated, and you are welcome to copy, modify, and distribute it under the terms of the GNU AGPL, which includes providing credit and a link back to this page.
See this StackOverflow question for details about the two types of email subject line spam emojis and how to exterminate them.
And for much more information see https://lurkertech.com/emoji and the comments in the source code of the userscript at https://lurkertech.com/emoji/gmail_subject_line_emoji_roach_motel.user.js