If you would like to ban a specific user you can do this from the Social Network Channel. There will be an "x" at the top right of a user's post which reveal a "Ban User" Option. Users can be removed from the banned list in "Settings" -> "Banned Content".
To block specific keywords from ever showing up in a feed you will need to go to "Settings" -> "Banned Content". Keywords can be added in the "Banned Words" field and should be seperated by commas. Once you've entered the words be sure to click update. When words are banned any posts witch contain those words will not show up in any channels you create.
This is also where you can unban a previously banned user.