He was sent to prison so that he didn’t rob any more banks.
He was sent to prison so that he couldn’t rob any more banks.
He was sent to prison so that he wouldn’t rob any more banks.
When so that expresses purpose in the past, we normally use a modal verb such as could/couldn’t or would/wouldn’t: ‘I spoke quietly so that I wouldn’t wake the baby.’
Here, being in prison made it impossible for him to rob more banks, so we use couldn’t, not didn’t.