If during the academic year you decide to stop studying, it is always possible to de-register. You can submit a request for de-enrolment in Studielink. Read all about it here: De-registration & Refund.
As long you are enrolled in higher education, you are entitled to student finance. Therefore you have to remember to cancel your student finance or loan yourself when you submit a request for de-registration. You can do this via Mijn DUO.
You must also deactivate your public transportation card at a pick-up point. This will not happen automatic, nor will you receive a notification from DUO. So stay on top of it and do it on time, because if you do not, or if you are late, you will be fined. Read more about it on the website of DUO. (Dutch language)
If you stop your student finance in your first year before 1 February, you do not need to repay your public transportation card and additional grant, as long as you will not continue your higher education study programme.