Stop Input Into Worksheet Based On Yes No Response

I am trying to work out how to stop input into a document if a certain response is selected from a form in excel 2007. Basically, i need some programming so if a question is answered a certain way - the user cannot continue completing the form as it becomes "locked" and an error box comes up saying this is the case - and what action needs to occur. As the list is a data validation list - i cant just separate it out - so that if one response is chosen over another then the error message appears.

For example the question might be "Will you use contractors". If the answer is Yes, the cell turns red and an auto response fills the cell (VBA has already been written into the document for this to occur). What i need additionally is an error box to come up saying "This audit cannot continue as a Minor Plan needs to be completed" and the rest of the questions responses say something to the same effect.