My landlady upstairs has a 10 month old pomeranian puppy. He's not very well trained and he isn't fixed yet. He is displaying obvious alpha behaviour around her other dog, and unfortunately my landlady is not a super responsible pet owner and isn't really handling him properly. I allow her cat down in the basement as he doesn't cause any problems and he prefers it down here. However, occasionally he'll leave the door open, and then the puppy will come down here and poop on the carpet or on my jacket if it is on the floor. From what I've been told he is doing this to try to be dominant, and it happens every time he has the chance, in roughly the same spot.
I'm wondering if there is any advice to be had about how to make him not try to act dominant around me or generally stop this behaviour. I've talked to my landlady multiple times and she generally tries to keep the door closed, but he still gets through once every week or two and it's really frustrating. I'm thinking of putting up a baby gate to block the stairwell, but that's not really a proper solution.