In TS2 when you jail someone(or just block)He stays there until he dies,but in TS3,the person just dissapears after a game hour being blocked.

This is really crap,Because the other way to jail someone is to use TCE true and add them to the family,and it IS crap because your sim,that doesent know them anyway,still gets bad mood when they die.....

So anyway to do this without having to add to the family?

I've jailed people, and they stayed. Not lately thou, last time was around LN came out so I can't say has anything changed after that.

It is a bit silly. I had a sim going on a murdering rampage and he stayed in jail for 2 days.. like a fuking "time-out" or something. But I suppose killing isn't actually a part of the game. It's custom content. Sims would most probably go to jail for destroying public property or stealing(? - kleptos?)

If they are in the Criminal career, jail is a job hazard. (Although having great relationships with you co-workers minimizes it.) But apparently the game doesn't like keeping people locked up.

I have a 'jailed' Sim.
She's my test subject.
For, uh, testing?
AND SHE KEEPS GETTING BOOKS.
0_o

Are you using half walls or normal walls for the "cells?" Did you remove the door after luring the Sim inside the room? Did you make your active Sim start cooking a meal on the stove, used Moveobjects to move the appliance into the cell with the victim, then resume play, and watch as a fire happens? You could try this method, if you want. But if you are not trying to kill the Sim, then you can ignore my suggestion.

Hmm...Im guessing that it treats these people like how Sims 1 treated NPCs...allows them to telly out of there...that way, you won't be bothered by their yelling...

Uh... let me test my understanding. You want to kill off NPC's without having to add them to the family .... right? Well.. you don't have to add them to the family. Do the TCE, and if the family you're playing knows them, you can Trigger Age Transition and kill them off that way one at a time. Just lure them to the cemetery otherwise your whole town will have grave markers and ghosts all over the place. Or if you don't want to use your family, use an NPC/EA canned family, make them know everybody in town and do the dirty deed. This is what I did once.

So... if that's not your goal, then I misunderstood the question completely.