Just send an email with your file number that you like to withdraw. SINP will send a confirmation after couple of months.
Dear XXXXX:
We are in receipt of your request to have the above-referenced Application to the
Saskatchewan Immigrant Nominee Program (SINP) closed.
If you wish to re-apply to our program in the future, please visit
www.immigration.gov.sk.ca.
Thank you for your interest in the Saskatchewan Immigrant Nominee Program.