Really easy?

...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.