Along with all of the physical violence, we have some very nosy nerds. With a network of communicators available, hackers are a constant, and they are in your data files, readin' your secrets. If they can pull it off.
On the room assignments, all identified hackers are assigned a letter to denote how good they are. This is not IC knowledge, of course. The scale goes as such:
A - The uber l33t hackers. Typically not even human, or at least not completely human. If that post is not 100% unhackable, they're in it. They are also the only characters with the actual skill for 100% unhackable posts.
B - These are straight up hackers. They have a lot of experience and a lot of skill. They can hack up to 80% unhackable.
C - These are people with strong computer backgrounds in their home universes and probably some hacking experience. They can hack up to 60% unhackable.
D - They have some practice and some useful tricks. They can hack something that's 40% unhackable.
E - They figured out that hacking is possible and have the basics. They can hack something that is 20% unhackable.
Locking a post is not the same as being a hacker. Security codes are constantly being tossed around and shared. A character doesn't need to truly understand them to use them. The only characters that can manage to lock out all hackers ICly are the A rank hackers. Everyone else, pick a percentage that makes sense for your characters' computer experience. They can become more proficient as they get experience and learn new skills, but keep it realistic.
How unhackable a post is should go in the subject line, with the appropriate percentage. Consider this permission given for any hacker of appropriate skill to hack the message.
For game purposes, if there is a post that you just don't want a hacker to read even though your character doesn't have the skill to keep them out, set it to 'Do Not Hack'. That just lets the other writers know that their characters need to stay out of it for plot reasons. Probably just didn't notice the message or were busy.