...then you're just going to be buttfucked by the ones who get up to mischief before they resign. You should have the ability built-in to recover from whatever they do, whenever they do it, because the worst damage is done by the insider you never suspect.
I'm generally in favor of the idea of that once someone submits a resignation, you might as well just tell them they don't need to come in. They can't get anything meaningful done in two weeks anyway and if you "need" them to explain what they do/project status/etc, then you're doing it wrong anyway and you won't find two weeks nearly enough time to get caught up.
Plus, what kind of leverage do you hold over someone who quit and has a job, anyway? Short of criminal behavior, you've got none. I've known a couple of managers at companies I worked at who were total assholes to employees who left, demanding extra work, tons of documentation, etc. It baffled me why the employees put up with it and knowing one manager in particular, I'm sure her employees hated her anyway and fucked up the work she made them do anyway. I know I heard rumors of shredded original billing materials and other documentation.
If you're desperate for a resignees information and talents, the best choice is to offer them a consultancy contract for real money. I think this gets people's respect, real quick. It shows you actually value their knowledge and skills (versus some bullshit words) and it buys you some leverage, since no work == no pay. But it has to be real money and guaranteed, "we might want you back for something later..." is no more believable than "let's have sex tomorrow instead." Tomorrow never comes.
The notion that there is some kind of Gentleman's Rules surrounding employment is over. Everyone knows they can be axed at the drop of a hat and most people feel no loyalty to their employer (or shouldn't, anyway) and could walk tomorrow. You have to be prepared now, not when they leave.