Lock variable value selections?

I'm fairly certain that the only way to go about this is via training or a QA process -- the way I've seen it handled in the past is to have the author (editor) hand off the completed asset to another team member or QA team (think branding, spell check, etc) to put a second set of eyes on stuff before it goes live. I'd recommend some kind of QA and testing process for everything that goes out, it's an easy way to avoid major mistakes and catch the little things that fall thru the cracks when you've got a lot of hands doing work.

If you're really suffering from folks going rogue, it might be a better idea to lock down the color options (remove the option/variable to change them) and make variations of the templates which use the different colors that are hard-coded.

Another option (which might not work as well with email elements, but maybe some things like buttons) would be to use CSS classes in the dropdown to control the styling of things like buttons. In this way, you could have a "Button A" and "Button B" option which would style a button two different colors by picking up a class in the <style> portion of the email but would not allow a hex code to be input and/or any other options to be entered like "Button C" or "red" b/c those wouldn't tie into the style sheet and the button would look obviously broken (the colors would default to something else that wasn't branded and stand out).

Here's a link to the documentation for reference -- I don't see anything in there to suggest a way to lock the options to only accept the dropdown values: https://experienceleague.adobe.com/docs/marketo/using/product-docs/email-marketing/general/email-editor-2/email-template-syntax.html#list