Why don't you name and shame them???
I think the simple solution is to either negotiate with the crew to keep them, or work with them individually on a mutual time frame.
For instance if my employer said, hey mate, we know you're leaving but we will be in the lurch if you leave to quickly. If you can give us another 6 months until we replace you, we will be very grateful and offer you $xxxx as a token of our appreciation. I would be inclined to work with them for being upfront and forthright.
I know for a fact a general aviation operator did just that recently to keep guys from going to the opposition. They all accepted. It was rumored to be in the vicinity of 20k cash up front.
Food for thought?
I know some operators are reluctant to go outside of their EBAs, but remember an EBA is the minimum conditions you can pay someone. There is nothing stopping you from going beyond it in every way for retention.