You have a complete history of a users tweets and a record of how long the person has been active, number of followers, textual analysis of the tweets, block/mute status with respect to other people, whether you follow ect. It seems like it should be really easy for twitter to put messages into a spam/harassment folder, never to be seen by the target.
...Really?
Edit: Not only would this alter a core and fundamental aspect of how Twitter currently operates but it would certainly be non-trivial to implement in an unobtrusive manner.
Who wants what blocked? Is there a generic 'spam/abuse' filter? Is it on by default? Are those tweets put into a spam/abuse section or entirely unviewable. What happens when someone sends a tweet that is blocked by the filter, are they notified?
Try to clearly define what metrics/identifiers you would use to determine a tweet is offensive and should be blocked and then call this implementation trivial.