The removal of seaweed depends on the location that it needs to be removed from. If it is possible, you can use a boat and a net. Run the net through the water to remove the seaweed.
If you got to your local "plant shoppe" You'll be able to purchase chemicals that both kill and remove sea weed. Or I've also heard the spreading baking soda over the infected item can kill the sea weed as well. It might be worth trying both options out. Good luck.
Seaweed can be killed by regular weed killers. If that doesn't work, try baking soda on it. Sometimes that will kill it. If you're left with a last resort, dump salt on it. It will dry it out.