Custom Validation

We have a field where we ask the user to enter in the City, State that is connected to the Google Maps API. Problem is, we have submissions where the user clearly didn’t select a location from the drop-down list. The user can enter in Massachusetts (as an example), where we need to know what city in Massachusetts (i.e. Boston, Massachusetts). To resolve this, I imagine setting up a condition where the first letters contain the letters of any of the 50 states would result in an error (Please enter a valid City & State). Has anyone written a custom validation to not accept certain values in a field?

Long story short, an error should appear next to the field if the user entered in any of the 50 states in the U.S. The form will accept city/state, but not just a state.

Thanks