Gently insinuating that I might have to switch them to another instructor, as I'm fed up of losing money (not prepared for the lesson at the start of their slot = time taken away from flying = poorer FI).
On a serious note, I might consider pointing out that such an attitude & lack of preparation is unlikely to go down well with the examiner during skills test. If they don't get into the habit of preparing properly now, they either won't be deemed ready to test (more cost for them) or the examiner will fail them (same result, due to retesting).
Or pointing out (if said student is going to hire in the club environment after PPL issue) that turning out without proper plans, plogs, notam checks etc will mean they won't get signed out by the duty instructor for said club.
Failing that, a remedial flight and gentle bollocking from the CFI might help.