there are different ways of canceling the import process.
For your requirements, you may not want to delete existing data in your POV if the file name does not match the active POV. Therefore you have to cancel the process before data clear is triggered
One solution that can help you to meet your requirement is to implement the following code in the "StartProcess" event of ImportAction Script when the file is not valid
If FileIsNotValid Then
RES.PcolScriptInfo.Item(1).ColFlds.Item("Cancel").varValue = True
RES.PlngActionType = 2
RES.PstrActionValue = "File is not valid"
The object PcolScriptInfo has different information based on the event (you can take a look to the API guide)
I Hope that Helps