Likewise. I have never heard the expression 'knock a habit'. I'm not sure it exists. 'Kick a habit' is a common expression, however.

never heard of 'knock a habit', so stay with 'kick the habit' instead.

'Knock a habit' is wrong. 'Kick a habit' means to stop a habit. "I used to smoke 20 cigarettes a day but last year I kicked the habit."