How to deal with a neighbour from hell?

I moved into my new place end of last year and below me my neighbour is on crack so is her boyfriend, I thought they were recovering but I've knowledge they are still users. Anyway she's always ringing my buzzer asking for money for ciggs & once even for a slice of bread after I said no to the money lol I always turn her down. She constantly argues and fights with her boyfriend, its annoying I can put up with their drama but when I come home & I'm tired I don't like to be bothered, I just want to be left alone to relax & call my family. How do I deal with her? She rang my buzzer three times and I just ignored it, but felt like giving her a right ear full of cursing. Also she's come upstairs and knocked my letter box calling my name. Flipping heck I'm a student & a single parent while she is single, no dependants & on benefits (I don't care about her financial status) how does everyone stomach her as its a small flat conversion of 5 flats & she's been there 3years. The police have been called on her boyfriend in march as he battered her & she tried to stop his arrest saying they were joking then came crying to my door, my ex told her I was asleep & said her face was bloody & bruised