Mechanical skimming is the answer--remove it and move the water with your fountain and aeration for the week and it should be okay.
I've been researching mechanical skimmers for some time and I've seen one work well but it's $6000
www.proskim.com I think it would take the weed out in a day or two and then you'd be all set.
I've inquired if anyone bought one to rent it out or perform the mechanical skimming with this product commercially--nothing yet--you may want to contact them to see if anyone in your area has one.
Other than that people have posted how to make a boom out of rope and swimming noodles and tie straps to pull the duckweed to the side.
I ended up treating with Sonar over a long period which is best to treat before the duckweed starts to appear. This isn't your case.
