The following could be placed in your list of work rules:
We hope that you will never find it necessary to resign. However, if you do, we ask that you give at least two weeks' notice. Proper notice allows us to prepare your final paycheck and separation papers, and preserves your record as a responsible and cooperative employee. In some cases, we may choose to pay your two weeks' salary rather than require you to work the two weeks. If you don't give the required notice, you may become ineligible to be rehired in the future.