I have the same problem although mine is usually powdery mildew caused by not enough water. I just pull off the affected leaves. It's quite a robust plant and spreads like mad so have a good go at it - it will spring up again.

Hi I have one in my garden which didn't flower last year but I am still hoping this year. You can get information on acanthus if you follow this link:
http://www.telegraph.co.uk/gardening/3324985/How-to-grow-Acanthus.html

Dont forget it has been a really funny year with weather, cold til May, then drought, these are pretty tough plants so I suppose its a waiting game, maybe until next spring.