I've added a field expected_date to the SO, When we create the SO, I check the expected_date. If expected_date is before today, I don't want to create the SO. I put the logic in beforeSubmit but I realized that I don't have the expected_date yet. However I see that I have the expected_date in the afterSubmit. Is there's a way to remove/cancel/undo/etc... the sales order at that point ?

Why not just leave the SO in Pending Approval status and then approve it once you have the expected date?

After submit is not event where you can block creating record. Where do you set value for expectDate? On the entry form? External. integration or other script?