As an aside, this all sounds very...pessimistic, I suppose...but you simply plan for the worst and hope for the best.
Usually there seems to be three types of people that you put on corrective action:
1. Those who will ignore you, make no changes
2. Those who didn't realize how poor their performance was was and really treat it as a wake-up call (ideal)
3. Those who take it as a personal offense and either a) start to perform worse or b) start to look for other work because they think the situation is beyond repair.
In 3a, these employees are usually pretty high-strung; I'd say this is the least likely possibility. If you've already talked to them a couple of times nonformally (and it sounds like you have), I'm going to guess they are either 1 or 3b.