Let's take the fat people hate subreddit. Okay, you ban every user and the subreddit itself. Some people make new accounts, and a new subreddit about hating fat people, so you ban all those too.
Did that convince anyone that their opinions were wrong? Absolutely not. If anything, convinced them, in their eyes, that they were onto something and had to be silenced.
All it's done is driven people with those bigoted opinions away from Reddit. These people still exist (until we start exterminating them I guess) and will continue to speak their bigotry wherever it is they do end up. All that's really accomplished is just cleaning the website of speech you don't want anyone to see. Kind of like sweeping dust under a rug - still there, can't see it though. Out of sight, out of mind.
Someone else's pest-problem.