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Actually, thinking as a moderator, this is probably worth tweaking one's code to disallow, since it could easily be used to link spam or abuse to another user's posts without them being able to report you.
A bit of code that checks if the action you took before blocking was replying to the same user and forces you delete all replies if you want to block them would be relatively easy and worthwhile to prevent abuse, imo.
I'll @jack and@twittersupport in case this could be of use.
You could also code it to automatically delete recent replies to a user you're now blocking (removing all replies within a certain recent time period like 10 mins), and have something that informs the user of this.
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