I might misunderstand your question.

I thought you want to be a full-time student after you finished your OPT. You generally can not cancel your OPT application. You can try by writing to CIS but a withdrawl is far from certain. Once full-time OPT is approved, you generally can not terminate it. You can change your job or hold multiple jobs at the same time. And you can transfer to another program within 60 days after your OPT is expired.