1
general::
Act more forcefully to regulate, repress, or restrain. For example, The police cracked down on speeding. [1930s]
American Heritage Idioms
2
general::
Phrase(s): crack down (on someone or something)
to put limits on someone or something; to become strict about enforcing rules about someone or something. • The police cracked down on the street gangs. • They cracked down once last year too.
McGrawhill's American Idioms And Phrasal Verbs