I love duckweed in my betta tank. I just scoop out the excess anytime it starts blocking too much light from the plants below. Because it IS a serious invasive species, however, the three best ways to dispose of excess are:
1) Find a friend with pet goldfish. Duckweed is a nutritious and tasty snack for them. Best by far, IMO. (My secretary has goldfish at home so I just give her my excess in a little tupperware container, everyone is happy)
2) Find a fellow aquarist who is looking for some for their tank. (sometimes you get lucky, but not always, and this could require shipping so less desirable IMO); or
3) Put it in a safe container with water (I like my Pyrex glass measuring cup) and microwave the hell out of it for 5 minutes or so to kill it thoroughly. THEN toss it in your flower bed to decompose into plant fertilizer. You want to make sure there's no way runoff could carry live duckweed into a water system and cause problems in the local ecosystem.