Is it allowed? We can’t see or control which userscripts people make, so yes, it’s allowed, providing it isn’t somehow abusing a loophole in the system or causing a sufficient load on servers.
Downside to such a script however would be if the person you’ve blocked tries to talk to you, whether it be something important or to resolve whatever issues are between you two, you’d be unaware and the only reasonable solution would be to give the script to the other guy so they don’t have to see your messages either..
But it’d still cause a loss of context when reading forums as you’d still be able to see people responding to their messages, but not the message they’re responding to, or responses to their messages.
tl;dr: I’m not a huge fan of blocking functionality, its only applicable function is to stop people from harassing you, but its often misused, so Personally I’d discourage use of such a script without a good reason.
