"De politie wil niet dat mensen misdaden plegen."
Translation:The police do not want people to commit crimes.
"Dat" is used as a conjunction here, it doesn't refer to "mensen".
The literal, word-for-word translation would be: The police do not want that people commit crimes. But that is stilted English, instead we say "... do not want people to ...," which is a grammatical construction that doesn't exist in Dutch.
Although this noun is singular in Dutch, 'the police' is always treated as a plural in British English - so 'the police are outside'. A police officer, a policeman/woman or indeed the police force are all singular - not necessarily logical but there you are. And you might also say 'the police force are' even though it sounds a bit clumsy and it's much more likely that you'd say 'the police' instead. Mostly, you would be referring to a particular organisation like the Yorkshire Police Force if you used this form....