Rinse the pad under running tap water for an extended amount of time, plus scrubbing and squeezing with your fingers. Do this until water flows smoothly through the pad. Dont worrry about the leftover chlorine; the amount is far to minute to harm your tank.
Rinse it in dechlorinated water if you're worried.
-Lynden
P.S. dont use soap, vinegar, or anything but water.
Thats what i've tried to tell a guy at work, because from all that i have read, if you leave sponges in your aquarium to get dirty, they collect lots of nitrates, so sponges should be washed under the tap.