writing a validation rule for a standard object

on activities(task object) we have a standard field called "subject". I created a custom field called Subject Topic. I moved the subject field to the bottom of the page layout to be used only if a person is setting an open activity for a later date. I created a workflow rule so that everytime someone enters a picklist option on subject topic it will automatically update on the subject field. the problem is now people are changing the subject field to different options.

I'm trying to figure out how to create a validation rule so people cannot change the subject field and it must always stay the same as the subject topic field. Any suggestions???