I was able to get rid of small areas of Spirea (in Eastern Washington
it must have been S. douglasii) and other rhizomatous woody shrubs
with good ol' fashion pig-tractor techniques. I used three, 10ft
long, heavy metal gates, wired together and anchored with T-metal
stakes -forming a triangle that I slowly flip-flopped-around the site
as the pigs rooted everything.
They made a big mess in the wetter spots -that really took me a long
time to repair (use caution -maybe start later in the summer?) -in
about a month the pigs were able to get the rest of the area to a
point where I could eventually manage it.
I have since used Spirea successfully in erosion control and riparian
restoration.