You threaten the employee that does this with termination papers 
Seriously, I don't know how you can avoid such changes. You can help by creating locked layer or maybe a read only external reference. Normally it is enough to have policies in place to prevent "shortcuts" just to throw the job over the fence for the next poor sap who has to figure out what happened.
If you are doing a check function, however, you can probably toggle the symbols and see if this was changed. I'm sure that certain modelcheck functions can throw warnings too if needed. I am not one that believes in significant automation so I might be the wrong guy to comment.