A character can only hack an entry if it's said to be hackable. If nothing's specified (for example, if I write an entry filtered to Fye), then no one can hack it. You also should take into account the character's intentions.
Someone like Sakura would be less likely to hack something, because of her kind nature, but she might do it if she was really, really worried and wanted to help out.
At the same time, if something is said to be difficult to hack, someone smart (like our Death Note charries :3) would probably be able to hack it, while someone else might find it impossible. Similarly, something made private by a very smart chara would be a lot harder to hack than, say, something made private by Mokona (not that Mokona is stupid, but she's all cheery and cute and probably wouldn't even mind if someone hacked her stuff).
Try to make sure you specify how easy it is to hack whatever you filter, or if it's hackable at all and/or by whom, to ensure that charas you wish to read it can, and charas you don't want to can not.
Any questions, just comment here :3