Here’s a procedure that will clear unprotected cells in the active sheet. Place it in a regular module in the workbook.

Sub ClearUnlockedCells()
  Dim r As Range
  Dim iCalc As Long

  Application.ScreenUpdating = False
  iCalc = Application.Calculation
  Application.Calculation = xlCalculationManual
  For Each r In ActiveSheet.UsedRange.Cells
    If Not r.Locked Then
      r.ClearContents
    End If
  Next
  Application.ScreenUpdating = True
  Application.Calculation = iCalc
End Sub

 
Don’t use a cell as the trigger, open the Forms toolbar, and stick a button on the sheet. When the Assign Macro dialog pops up, select the above macro. Then change the caption on the button to something like “Clear Inputs”. Then protect the sheet.