Rub the board down with a cut lemon and wipe with a paper towel.
Mince a few cloves of garlic on the board, than mash the mince into a paste by rubbing them with the side of your knife. Rub them all around the board. You can reserve the minced garlic for another purpose if you like.
Clean the mashed garlic off the board, which should -- by then -- smell of garlic and not fish. First use something very mildly abrasive, like a Scotch-Brite "green thing" or a pad safe for non-stick cookware. Clean the board again with a damp towel or sponge and make sure that it looks and feels VERY clean.
Wipe down with ordinary kitchen disinfectant, or -- better still -- mix your own using ordinary household bleach. If you do mix your own, store it in a spray bottle and use it "as needed;" every time you cut a chicken on your board, for instance, but at least every few days; and, of course, it's great for counters.
The disinfecting process itself won't mask or eliminate smells, but kitchen disinfectants are by and large fairly powerful cleaners; especially bleach.
If you like, after disinfecting, you can repeat the lemon rub.
To some extent this process will get rid of the old smell; but mostly it's going to cover it with less objectionable [ahem] aromas.
Finally, oil your board with food-grade mineral oil (that's the stuff you get at the drug-store which is used as a laxative, it won't necessarily be labeled "food-grade."). Your board probably needed an oiling anyway as most home boards do. And even if yours didn't, it won't hurt anything.
If you choose not to oil after disinfecting, at least wipe your board down with a clean, damp rag (or sponge).