Letting go of an OFS. It's tough, but I'll give you simple advice.
First, tell them early, so they have time to find another job. 
Second, if they did well, give them a positive review on OnlineJobs.ph. 
Lastly, if possible, offer them some money when they leave.
It doesn't have to be hard. Just be honest and nice. By doing that, you're not just handling a situation; you're making things better.