Is there a way to always ignore a particular wifi network on your iPhone
The only way to do this would be to set "Ask to Join Networks" [sic] to OFF in your iPhone wifi settings.

I find this to be preferable. The main consequence is that, when you are in some place new, you'll have to manually select an available wifi network. But once you HAVE joined a network, it'll automatically join it when next available. And (more germane to your question) it WON'T ever join a network that you haven't explicitly joined.