My most basic description of a sponge player is the type that soaks up bets like a sponge, and then fires back at any sign of weakness. They never fold, or rarely fold, and can take you off your hand alot with a big bet on the river. Its especially frustrating when you have a sponge on your left and can never get position on him. I need strategies for beating this player, what are your thoughts?

They are tricky to play if you are in the habit of always making continuations bets (it's a strategy I adopt when I have position on a player who bets out hit or miss - 2/3 of the time they miss but bet anyway, so I raise the mimimum).
I lot of them call the min raise but most of them don't want to fire the 2nd bullet OOP so just check or make a 'blocking-type' mini-bet. Bet the pot back at them and watch them fold. Very profitable. Those who have hit hands tend to bet the pot on the turn too - it takes real cajones to bet out twice with air into a solid player who has just check-raised you..

The best answer of course is to be willing to let go of hands that miss the flop when you are OP against him - if there is no fold equity why make a continuation bet with junk?
Wait until you do get a hand and then they just beome a calling station you can lure into betting over the top of you by slowing down on the turn..

If you must try a run a bluff OOP on a player who 'never folds' (not that I would suggest it) you could start check raising him - the turn play wil be very tricky though - do you check or bet out? Guess it depends on the board and the hand you are trying to represent..