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.

Federated twitter blocking where people can choose to outsource the blocking to people they trust would be pretty cool. Kind of like AdBlockPlus lists.

This exists: https://blocktogether.org/

You can farm out your actual tweets, farming out blocking sounds like an interesting idea!