You can cancel your registration within 60 days without paying any premiums.
After 60 days, you can cancel it any time if you don’t make any claim. But the cancellation will be effective at the end of the calendar year, so you have to pay EI premiums for the entire calendar year.
Once you have claimed any EI benefit, you are locked in and you can no longer opt out. You will have to pay premium for the rest of your life on any self-employed income, regardless of any change in the nature of your self-employment. So think carefully before you opt in.