Pretty hard - ever hear the Union term of "scabs"? In a nutshell, no matter how vile someone is, there's always someone out there who prefers profit to morals and will gladly take their money.
Even if you somehow managed to get total, unanimous boycotting by every single person and business in the region where the landlord lived, The Rich will still find ways to get products and services anywhere. The easiest answer, obviously is to send your flunkies out to do everything for you - pick up some chinese take-out, drop off the dry cleaning, pick up the kids from school, whatever. You can't exactly boycott the landlord's paid servants and help and prevent them from using your services, can you? If nothing else, you don't know who they are.
No, scumbags like this thrive because the system in place makes it almost impossible to act against them. Even if you get them arrested, they go to a private luxury jail cell, eat caviar and drink champagne, and get out early on good behavior. Meanwhile their paid goons go to town ruining the lives of everyone they can manage who had a hand in getting them arrested.
Being a landlord means you make money doing nothing. It is the closest thing there is to being a feudal lord in the modern world, receiving a portion of the labor of others simply by dint of being their ruler. The housing you own does you no good - you have no need of it, you can make no use of it. It is only worth something if people live in it. And people HAVE to live somewhere. So you simply own as many houses as possible, collect a substantial portion of the incomes of countless people, use a fraction of that pure profit to hire contractors to perform the bare minimum legally required maintainance, and pocket the rest without lifting a finger.