The Bitter Employee

My years in the industry has taught me a golden rule - never try to retain an embittered employee.
I have observed my colleagues and also myself to fugure out that once an employee gets disillusioned about his/her career in any organization for any reason, it is hard to win them back. They will complain about reason X, then once that gets satisfied will move on to reason Y and further onto reason Z. The company will go out of it's way to satisfy the bitter employee's whimsies only to end up losing him/her at a later date and at a greater cost to company.
However, the first thing to do is to differentiate the chaff from the grain. There are 2 kinds of 'whiners':
1. The kind who whine and never mean it - This is the comparatively harmless variety. If they are otherwise good employees, it makes sense to just lend a patient ear and reward them on and off as an acknowledgement of their performance.
2. The kind who whine and mean it - Unless they are exemplary performers, dispose of these employees with no mercy. Their productivity will plummet to zero in no time and if left long in the organization they will probably influence other non-whiners negatively. The company might end up losing 10 people instead of 1.