The concept is extremely interesting for me, as it perverts one of the main ideas behind good design, the idea of the designer always looking for a design that makes the use of the object easier and more pleasant.
The essence of this idea is that, at some points, certain public spaces are shaped in a way that restrict the possible uses that citizens can make out of them. In this context, unpleasant design is oppression.
Think of, for examples, the spikes situated in the storefront of many shops out there. They are explicitly designed so you can’t seat in that space, blocking the view of the products and potentially provoking the wanderers to not to enter the store and spend their money on it. That is a common example of unpleasant design. Another common example would be the benches situated in public spaces that have handles in between the spaces with the sole purpose of avoiding homeless people lay down and sleep on them.
There is many questions that arise when studying the products of this design. Is it ethical to design, as a designer, a product that you know will oppress other people, rendering them unable to do things that should be acceptable?, is it a good design if the goal is completed, even though this goal may be unethical?, who is the moral responsible of this designs, the designer or the person that sponsors the design and dictates the goal?, should the designer reject this kind of jobs, as they are against one main goal of good design?
The answers are difficult. There are, nonetheless, ways to oppose this. Apart from the possibility to protest against this in a bureaucratic and official way by going to your town hall (or any other entity responsible of the public spaces in your surroundings) and writing a request to change this antisocial tendency, you can always hack this perverted design paradigms and get back the public space for the public non-restricted usage, as it should be.
Remember that performance is another way of protest.