Try scolding him.
If that doesn't work, try feeding him foods enriched with vita-chem or selcon and fatten him up to where he does not need the protein or fat he is getting from the SPS slime.
ALOT of the blennies, gobies, and wrasse pick at SPS in the wild, a good majority of SPS dwelling gobies eat the polyps of SPS when they attain a certain age.