The error messages available in the clipboard pages are being formatted before displaying in the pyCaseErrorSection and this formatting is happening from the Engine API. So i don't see any option to remove the ** characters from the error message display as it is being added at runtime of the display.
UPDATE: Hello - I've found out how to get this to work. I created a message rule under Create > Technical > Message. Note that this only works if the message rule is created off of the @baseclass. Then inside my edit validate rule I was able to reference my message rule like this...
ErrorEmailFormatWrong is the name of my message rule. It won't be referenced if @baseclass isn't used as the base of the rule. But I did @baseclass and now it is working for me and removing the ** from the beginning of the message in the validation.