preventing event withdrawls after event started

Right now, CR allows me to prevent withdrawals from an event within a certain number of hours, but it has to be minimum 1 hour.  I want to be able to set that to 0, but CR doesn't allow it.  If someone has to withdraw from an event 45 minutes before start, I want them to be able to do that.  If someone wants to withdraw from an event after it starts, I don't want them to be able to do that.