The first thing to do is strip strip strip. I think hemp can really hold onto detergent like you wouldn't believe.
That may or may not work, but it's the first thing to try. I periodically deal with hemp stink, and usually have to resort to a bit of bleach. It hasn't hurt my hemp any to do this. I just have to rinse it out REALLY well afterwards!
We get occasional hemp stink, but not from buildup. Something about the ph in our water/ph in hemp/ph in urine... I have no idea. All I know is that there is no buildup yet the hemp stinks to high heaven sometimes when DD pees.
I used bleach occasionally at first, and it was VERY effective! But, I noticed the hemp in some of my AIOs wearing out. I wasn't sure if it was the bleach or not but I stopped using it anyway. Now I occassionally use 1/3 cup baking soda and 1/4 cup vinegar in my cold soak and it works wonders. No more hemp stink!