How do you handle disputes on your services?

This is more like return policies that are out in place for physical goods. If those that purchased them are not satisfied they can ask for refund. This can happen with online services as well, when clients express their disapproval of the service completed.

How do you handle this when it happens?

I try to stay calm and open-minded during disputes. Understanding the client's expectations and finding a reasonable solution is usually the best way forward.