Yes, you can allow submitting authors to withdraw the abstract / submission so long it is incomplete.
Please refer to this system option:
1.04 Profile Parameters -> Profile Settings* tab -> Allow Abstract Withdrawal -> Yes
Please see this screenshot: https://www.screencast.com/t/d6YZXaYv8
!Note. Should an author for a Completed abstract wants to be withdrawn, the process would be that this author emails you asking to withdraw their Completed abstract(s) and a project administrator puts a status (e.g. Withdrawn by Author) and moves that abstract to Incomplete list.
Should you have any further questions please don't hesitate to contact us by submitting a new request.