In general
withdrawn submissions are not considered during the reviewer assignment process.
But when a submission had been assigned to a reviewer before, the assignment will not automatically be deleted when you withdraw the submission.
However, administrative users, conference chairs or track chairs (if the function to withdraw submissions is enabled) can decide whether they want to delete review assignments and reviews when they withdraw a submission from the conference.
Please go to:
Overview => Submissions & Reviews => List of Submissions In the column on the right-hand side "Actions" select "Withdraw".
For the option "Unassign & Delete Review" tick the selectbox "Unassign & Delete Review" (see image).
It would be a good idea to inform the reviewers about the withdrawal and cancellation of the assignment.
Otherwise they could be confused when submissions slated for evaluation just "disappear" from their list.
Hints:
Follow the links to find more information on
the conditions for the withdrawl of submissions by authors and
how chairs can withdraw and delete submissions.