Yes it is so frustrating and a mesh cover from planting unril October is the only remedy I know.
For example I had bad leek moth last year, on virgin soil where no leeks had grown before, so there is little point worrying about chasing them out of your garden: new ones arrive!
You have to live with it, or mesh. If growing winter leeks eg Bandit, they bulk up in spring when leek moth is not around, though they can still be eaten as small leeks in late summer. I think its not worth cutting to low level as I find that damaged leeks regrow well from October, after caterpillars pupate, and with better results than if leeks are cut down.