|Assuming macros are enabled on your user's machines, put this in the ThisWorkbook module of your workbook.|
Private Sub Workbook_NewSheet(ByVal newSht As Object)
Application.DisplayAlerts = False
Application.ScreenUpdating = False
Application.DisplayAlerts = True
Application.ScreenUpdating = True
MsgBox "Adding Worksheets Has Been Disabled In This Workbook."
You'll need to hide and password protect the code to prevent users from seeing/disabling it.