I have expierence this before in my previous job as a Manufacturing Engineer but on the paper work side of processing the job. I know your frustration.
Advice on how to get it through their skulls that using bylayer is important...
Tell them to go into one of their worst creations and change the colour of the text layer to yellow. Then show them how much easier it could have been if they had simply drawn everything BYLAYER. Yes, they can use QSELECT, etc. to select all of the text, etc. but the standard is there for a reason... not just 'because'.
Quick Select and CAB's routine will be your friends. I Cant agree more for a beginning step
My only other advice would be to send each and every single drawing back to the offender and tell them to put everything back to bylayer, then resubmit it to you.
Second Step. M-Dub, Jonesy will need the speak softly and carry the big stick of management. If she doesn't have support of management, she will never be able to get any changes done with the engineers. If you don't have the big stick you are wasting your time.
If they only do CAD occasionally and their skill level with AutoCAD is quite low and will likely never increase that much, chances are that they will never truly understand the benefit, so when they ask "Why is it so important to you?!" in their whiney voice, just tell them "Because it is..."
The ultimate solution would be to enforce the standards through Customization. That way the engineers can "design" <cough> and not worry about standards. And you wont have to fight the new engineers that come on board.
My 2 cent opinion from my experience. Convince management first then go after the offenders. Management may even allow you to bill some time to customization.