Use a mild example to stop a more annoying action...

If you want A to stop doing B action. But, before you can say it, A has gone on to do C which is more annoying than action B. While A is doing C and annoying you, mention that A should stop doing B (which is more distant now) and A would indirectly be led to stop doing action C, because A would be wondering and tracking back to action B and indirectly stop doing C... 

Asking A to stop doing action C would make you sound harsh because you are directly ordering the person to stop doing C. By referring to something past like B that A should stop doing does not sound so demanding.... By putting a negative feeling into A, A would not continue to do action C