Being constantly bullied

I live in a flat, my nightmare of a neighbour who lives on the opposite side of the building and floor below me who I haven’t met or seen since the incident I am about to mention decided to smear turd over my front door and carpet in the communal hallway. I am the last flat on the the top floor and still have no clue why he did it. I have been told he is mentally ill. I really hope it wasn’t his own he smeared over my door. But he has been purposely buzzing the intercom system to my flat, breaking into my mail box, stealing my parcels and calling the housing authority on me with some really absurd claims. Luckily my neighbour below vouched for me despite only seeing them rarely in the communal hallway. 

Literally just stand up for yourself. People will stop bullying you if you do. Be firm and assertive. Try not to pick a fight though. Just say things with a certain level of seriousness. Like if someone is calling you names or something just say something like " Oh I'm sorry, you must be confusing me for someone who cares ". This is just some thing that worked for me in high school. My point is just don't sit back and take it or it will never stop. Or just try not reacting at all to it. They will stop bullying you if they can't get a reaction out of you as well. Hope this helps.

completely agree my neighbour is exactly this… they have absolutely nothing to do in their own lives but to analyse ours… really annoying but have to learn to not let it get too me but easier said than done. Thank you for your comment it was helpful 

Thank you for your advice much appreciated 

eh id tell their landlord instead and get them evicted.

usually rental unemployed types who dont own their own home and who have too much time on their hands that do this.... anyone else doesnt care or take any notice of who their neighbour is or have any interaction with their neighbour as theres no need and they are too busy living their own life.

I would record all instances of unpleasant behaviour by your neighbour, keep a note book of times and dates. If you can, record interactions on your mobile phone - covertly. Then you will have evidence that you can either inform your neighbour about its existence, in the hope that this might give him or her pause for thought, or take it to your local council or police. If you are autistic the bullying would be considered as aggravated as it amounts to harassing a disabled person. Telling your neighbour that you will inform the local press might also have a useful effect. For all of this, you need the evidence first.