Unless they happen to be in the way of your ornamental or veggie gardens, perhaps it would be best to let them be? Granted they are not classically beautiful, I preffer to appreciate thier "architectural form"
But that is simply my opinion, it is your garden and as such are free to do as you lease.. that is the true charm of gardening, after all.
If you are insistant in removing them, the most sure fire way to do it (I expect a firestorm from the diehard organic gardeners for suggesting this) is wait until spring when the leaves are just starting to open, cut the sumac down to as low to the ground as possible. Drill a few 1/4 inch holes vertically down into the trunks, close to the outer bark and a few in the heart. Carefully pour about half a cup or so of a brush and ivy killer onto the holes taking all applical saftey precautions such as good rubber gloves, safety goggles and appropriate clothing ( not gonna actually reccommend a brand, new KY state laws prohibit reccomendation of garden chemicals other than fertilizers)
waiting until spring guarantees that the sap is flowing well in the sumac and the holes ensure that as much of the brushkiller as possible is absorbed by the tissue and transported to the roots not spread about to more valued greenery. Wait to see if the shrubs start sending out shoots, about a month to 6 weeks. If you don't see any, then you can assume the sumac are dead. If there are shoots, then you will have to start a shoot patrol and pull them out as soon as you see them. you will eventually starve the reamaining roots of much needed solar energy and they will eventually die off. THIS may take quite a while.
Got all that???