I don't know how it will affect the deck, but WD-40 will easily cut pine pitch. I use it when get that stuff on my hands. You might try a small, hidden spot for a test first.

I use rubbing alcohol to remove fresh pine and fir pitch from my glasses. It doesn't work so well on old pitch (as on my car). I would be cautious of the oil in WD-40 as it might tend to stain the wood. Come to think of it, turpentine is a solvent based on pine pitch, I believe. That might also work. Scrape up what pitch you can with a putty knife before you apply your solvent.