I dont see the ignore button on the forum so you must have written the name of the guy manualy to ignore him in your user cp. I never tryed the feature myself but maybe you just have to do the exact thing you did to ignore him. it like for mod, we have a stick/unstick option, its toggle between. I guess its same for ignore.;)
Never used it... never really looked at it even. You might want to try putting the guy in your buddylist. Just a wild guess, but it might work. After all, it is the exact opposite.