Your recipients make a good point, and the graphic symbols which are used to quote previous messages should be removed. For short messages, this is not a painstaking process. As you found out, the changes cannot be made in the original message. After you have pressed the Forward button, you now can take full control of the text and edit it to make it look good. Even though the information is not original, it still reflects on you.
Some advanced users bring the message into a word processor and change it to suit.
I recently started using a Freeware program, eCleaner, which has been around since the days of Windows 3.1. There will be a good explanation of the program and links to various download sites. Select Clean from the menu or toolbar. You can now copy and paste the results into your outgoing message. There are several other options such as correcting word wrap and changing fonts which you may select.