To allow or disallow volunteer assignment self-cancellation, you can define your own rules. Whenever a volunteer tries to cancel an assignment, VSys Live checks these rules:
If a person meets the criteria of the rule (People types, Volunteer types, Groups, Lists), VSys Live counts the number of assignments for this person with the status of "Cancelled by volunteer" or "Volunteer requests replacement" based on the Count... fields here. If the volunteer has at least the Maximum self-cancelled assignments within that window, VSys Live will show your Message... rather than letting the volunteer cancel the assignment.
Assignments marked as "Cancelled by staff" do not count towards the limits here.
If a volunteer meets the criteria of more than one rule here, she's checked against all of those matching rules.
A Maximum self-cancelled assignments value of "0" precludes the volunteer from cancelling any assignments meeting these criteria. It doesn't hide the "Cancel" link in VSys Live, but it does show your message here if they try to self-cancel.
These rules are meant to limit how many times a volunteer can cancel, not how close in to the assignment's start the cancellation is allowed. Limiting how near to the start time a cancellation can be made is done in VSys Live Settings.