I have member of my local forum that had this same problem and the shrimp was no help to them. at first it was working and whacked about half of them but it got to the point where there were still starfish but the shrimp just could not find them and the owner was just finding them and giving them to the shrimp in order for the shrimp to stay alive.
So what the point if your just going to find a few and then hand them over to the shrimp or just manually removing them?
I would try a wrasse sense they move much faster then a shrimp, aren't as shy as the shrimp and still can keep them once the shrimp are gone.
Or look into getting an:
arrow crab
Hawkfish
hogfish
some pufferfish (do your research first on which ones)