Establishing a block against another individual on Twitter helps protect you from annoying, harassing or bullying Twitter users with whom you don't wish to interact. Once enabled, Twitter does not notify the other user of the block -- but the individual may still be able to figure it out as there are a few telling signs that a block has been established. Furthermore, while the feature is easily enabled from the user's profile, it is not nearly as restrictive as similar blocking options on most other social networks.
Blocking and Unblocking
To establish a block against another Twitter user, log into your Twitter account and navigate to the profile page of the user you'd like to block. Select the small silhouette icon on the profile page -- it's typically located next to the "Follow" button -- and select "Block" from the available options that appear in the drop-down menu. The block will be established immediately. Should you later decide to unblock the user, follow the same steps to locate the "Unblock" option.
Limitations of Twitter Blocks
A blocked user will be prevented from following you, adding you to lists and mentioning or replying to you. If you're blocking a user who followed you previously, that profile will also be cleared of your profile picture and any of your previous Tweets. The block does not, however, keep the user from viewing your public profile page, nor does it prevent the user from re-Tweeting your posts.
A Telling Sign
Blocked users can still determine a block has been established even though there are no formal notifications sent when the block is put in place. For example, when the user eventually realizes you are no longer appearing in his newsfeed, he may go to your profile to click "Follow" once again. A warning will then appear stating, "You have been blocked from following this account at the request of the user." Should the user also attempt to mention or reply to you, the post will not appear in the recent Tweets list -- which suggests a block has been established.
Privacy In Addition to Blocking
Further restrict access to your profile by configuring your own Twitter profile to private after establishing a block against the bothersome user. Tweets that originate from a private profile are restricted from the view of those users who are unapproved, plus other users will not be able to re-Tweet your posts. To establish privacy, click the gear icon at the top of your profile and select "Settings." Click "Security and Privacy," enable the "Protect My Tweets" option and click "Save Changes" at the bottom.
Other Restrictive Actions
If a block doesn't seem adequate or appropriate, you can also report or unfollow other users. Unfollowing a Twitter user prevents the person's posts from appearing in your newsfeed -- without notifying them of your actions -- but still maintains the ability to view the user's profile and available Tweets. Reporting a user or post for spam not only blocks that individual from following or replying to you, but also initiates an investigation -- with possible account removal -- by Twitter.