So what, you just let them hang around the office for while? Maybe poach a few employees? It might not prevent any damage that's already occurred, but there's no reason to allow more damage.
Work out an end-date with them that will allow for a comfortable transition & knowledge transfer. Perhaps that's "immediately" if someone else can take over their job, or if they're not interested in staying and helping you. Perhaps it's "2 weeks from now." But generally assume good faith, because if they wanted to screw you, they would've already done so.
