I believe the only options are to try and dry out the block (for months, in a hot & dry environment with fans), or remove it. Many sailors concerned about weight do remove the block completely, but it's illegal to do so if you plan to race in a Hobie class. Originally, these blocks were contained in plastic bags to keep the water out and keep the fumes from eating the foam when the glass was first laid up. The bag eventually disintegrates, which leaves a mess in the hull, and allows the foam blocks to absorb water and add a lot of weight to the boat. As far as putting foam floatation back in, I haven't heard of any good solutions or anyone attempting it. Not sure the block that's in there would float the hull at this point, and any other foam blocks are likely to be susceptible to the same problem.
If you want to drive yourself nuts, I believe the mythbusters have had some luck floating boats with ping pong balls!