My employee gave me their verbal resignation yesterday and wants to come back today – do I have to let them?
If it was a “heat of the moment” resignation, usually yes.
Resignation is unilateral, in that you don’t need to “accept” it for it to be valid.
That said, if there has been a discussion, tempers got frayed, and your employee storms out saying “you can stuff your job!”, then you should give them the chance to cool down.