How to remove dirt from inside green/spring onions?
This occurs pretty often with leeks as well.  The procedure there is to cut open the stalks lengthwise - i.e. one cut from top to bottom along the long axis.  This allows you to fan apart the layers to ease out any trapped dirt.

You could do the same thing with your green onions, assuming you don't mind cutting them lengthwise.  If you don't want to cut, you could try soaking them in a sink or pot full of clean water for a half hour or so, agitating them every now and again.  The water should get down the stalk and at least loosen up any trapped dirt.