hullexile, this is an excellent point. I would expand a little on that.
Not only do you have to document everything (because the one with the most documentation is very likely to make a better case.) You have to make it public knowledge by writing him\her out with HR. If your HR manager does not want to help, you can actually force the issue by letting your direct report know that you are setting up a meeting with you, the employee, and the HR manager so you can discuss the current situation. Have your direct report acknoledge the issue so that you can cover your back.
Next, go to the HR person and advice her that you need her to set up a meeting with the employee and that you need her there. Pursue by giving her 2 to 3 choices of possible time for the meeting that she can select from. "This are the time I have availlable. Please make it happen either 1 or 2 or 3" What ever those times are.
When the meeting takes place, you need to be able to keep you kool and just state the facts as accuratly as possible and propose a solution by setting up measurables that are directly tied to the employees job description and make the employee aware that failling to achieve those measurables may affect his\her performance evaluation. Then give specific time frame on when you expect that to take place. Finally, ask the employee to tell you and the HR person how do they intend to achieve these measurables. Then hold him/her to it. (If you give someove enoght rope to hang themself, they will eventually do it.) (Also, please note that in this process, the HR is really only a report writer. You are not asking for her opinion. You are just asking her to document the meeting and put it in the employees file.
Once the employee deviates from the plan, you may write him up in the same manner but this time escalate the severity from may affect the performance evaluation to may affect your future with the company. Then finally from may affect you future to will result in termination.
If you follow that process, HR doesn't really have much choices and you are backing yourself and the company up in case of back fire from the employee.
This maybe slower or faster depending on the employee's hability to adapt to change.
If you need a faster process, you can consider couple other issues.
1) Have you heard of the saying: "A barking dog does not bite" In other words, many people can threaten to sue. The reality is that very little of the have the knowledge and\or the means to do so. (It is expensive to use legal services and unless it is a major case, no lawyer will take a contengency case like that.)
2) Last consideration and I beleive it has already be mentioned. HK is a very liberal economy and unless you made a gross mistake and really abused of your authority with the employee, you don't have much of a risk for a law suite as HK Laws tend to favoritize the employer rather than the employee. And once again, no lawyer would want to trial a case like that.
I hope that helps a little. Good luck