Block

The concept of "blocking" another user of a service you're on is not new; in fact, it goes back to the pre-web days of CompuServe and GEnie. It's simple, you want someone to go away, you block their access to what you write. In most cases, a block will work both ways, so that they don't see your stuff and vice-versa.
The actual effects of blocking another person vary from platform to platform. Take Twitter, for example. If your tweets are "public," that is, they're freely available in the public timeline, a user you've blocked can still read your stuff. They can't reply or use the "new" re-tweet format, though. Your block also works on client software like Seesmic and Tweetdeck. Thing is, though, your tweets are public, so the blocked user can go directly to your page on twitter and read the stream. They could also copy/paste your tweets into their stream to re-tweet. In other words, a user can still talk about you even if they're not talking to you.
You have a limited defense to this on Twitter, and that would be to "protect" your tweets by removing them from the public timeline. Once you "un-follow" someone, they can't see your stuff on any platform. The catch is that this is an absolute--when you go private in this manner, nobody but your followers can see your stuff. This is quite limiting if you're using Twitter for business or to meet new people.
Still, blocking is usually effective with all but the most ardent twitter-stalkers.
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