A good method is the first dust the flies with your calcium powder and then sieve the powder away with a vey fine mesh sieve. This will hopefully dislodge the mites. Also using mite papaer helps - as well as washing your hands after handling cultures.

On the plus side most dart frogs will readily take mites as a food source - my azureus love them so I keep a few mite infested cultures in the shed for that purpose (well away from my active fly cultures tho)