I don't think I've ever come across a way of getting rid of them as they lay their eggs and then fly away and it is the grub that does the damage. This is why all mentions of protection are for keeping the fly off in the first place.
If you have this problem then good luck to you, I think you're going to need it!

You could try spraying with a mixture of washing up liquid and water, but unfortunatley if it's established there is little you can do about it.....if it already has a good grip chances are the maggots are already feasting on the carrots growing in the ground.
Prevention is better than cure, try companion planting....plant garlic, onions, shallots or leeks next to or in between your carrots....this is supposed to work because the strong smell confuses the carrot fly!!
Or put a barrier/net around your carrots....about 2 feet high...because carrot flies only fly less than 2 feet above the ground....even fine net curtain will work!!
Or you could always try a more resistant variety such as 'Resistafly', 'Sytan' and 'Fly Away'
Hope this helps...

Try building raised beds. Angels is right about them not being able to fly over a certain height.

Horticultural fleece in a "tent" shape over and around them from the day you plant is one of the accepted ways of keeping the little bleeders out. Unfortunately once you have them there isn't really any cure. (I've been checking with my fellow plot holders down at the allotments) Their comments would get me kicked off this site! Lol