Batman should have killed him decades ago. But, since he's the single biggest hypocrite in comics, he'll tell other people about how it's wrong to kill while letting a mass murdering psychopath run loose offing entire families.
Of course, even killing the villain is not a good solution in comics... bad guys come back all the time. There was that hilarious scene in X-Force when Wolverine complained about it.
Some one ought to have snapped and done him in by now
Jim, Bullock, One Of His Inner Circle (One of them should have done it for him) An External Actor (The Spectre, The Demon's Head, Intergang, Checkmate, Harley's Dad)
Yeah the problem is with any prison he will always escape eventually. But as pointed out about killing him even that doesn't stop characters coming back. He'd probably do some deal with Neron or somebody and come back worse than ever.
This is what I really don't get. Even if Batman doesn't snap somebody else would have by now (was the Spectre's non-intervention with Joker dealt with somewhere or am I thinking of someone else?)
It's also been discussed before - eventually some victim would be related to somebody very wealthy, or enough victim's families would pool money, so that Deathstroke, Deadshot, AG, or whoever would just be hired to do the job.