Nothing can prevent terrible people from using an open source project. Meanwhile, Reddit doesn't do much of anything about this problem either. For, example /r/metacanada exists, and white supremacists from there are also moderating /r/canada subbreddit, Reddit hasn't taken any action on that. At least the developers of Lemmy are very clear [1] about their stance regarding nazis, that's more than I can say for Reddit. It's also worth noting that Mastodon has millions of users now, and it clearly isn't attracting the worst people. In fact, I find it's a far healthier and friendlier community than Twitter.