With extreme difficulty. I've just got rid of some after 5 years of having the dreaded stuff....i had to empty the pond and leave it dry for the winter.
If you can't empty the pond as you say, it's just a matter of netting it out every day or week.
Some people say that grass carp will eat duckweed but i don't think it's a sure thing
We have a device which is like a long length of wood with nails stuck round the bottom probably two or three on each side of the wood a couple of inches apart - you then plonk it in the pond in the weed and twist it round and round and all the duckweed collects up abit like getting spaghetti on a fork and much easier than trying to fish it out with a net.
we also had a pair of ducks on the pond one year which did eat all the weed but thats not to be recommended if you have any small fish - all ours are koi are so big they were more likely to eat the ducks than vice versa