Frustrating and annoying as it is, you could have bigger problems. I suggest the best thing is to not show that it bothers you. Just take in your mail and put the junk mail into the recycling.
Or, do something that does not look targeted such as providing a recycling bin next to the letter boxes as a convenient service to all residents. Anyone seeing your pair post the junk mail in your box will know they are doing the wrong thing, even if the background is not known.
Whatever you do, make sure it would be seen by other residents as dignified and not stooping to the childish levels of these irritants.