hook a chain around the individual bush and fasten to a truck and pull them out. Or use a good come along and winch them out.
These bushes have quite a bit of determination and desire to live. You will need to remove the root and turn the soil and destroy the area as much as possible. Chemicals will only render the area barren for future planting. i would steer away from that area.
I would pull them out, then turn the area and not do any work there for a year or two until I have removed all the returning shoots. These shoots have roots and each shoots root MUST BE REMOVED.