I might be wrong...
But isn't carbon (ie. Charcoal) a great way to absorb neutrons?
It also absorbs impurities in water, but I'm not sure you would want to use it for that after it has been used in a fusion reactor...
Best to sink it to the bottom of the ocean probably...Where the carbon would remain sequestered for millions of years.
Carbon Aerogels might work, but I don't think they would sink...