More than one time but less than thirty? I suggest that a points system could be useful.
As in most places like that, a relative handful of criminals committed nearly 95% of the crimes. On a quiet night, they would go through the files and decide which ones were due for an accident or a mysterious death in a brawl or the like.
A few days later, said career criminal would mysteriously end up dead in some back alley or the like. Of course, an investigation would be conducted and, of course, they would have to close the case due to lack of evidence.