First, wait until frost is a thing of the past. Keep scraping farther back on the branches. Cut right where the inner bark shows moist and light green--dry and brown won't sprout. Sometimes the most common species of Acacia used for mimosa flowers (A. baileyana) refuses to sprout from trunks more than 3 cm in diameter. Alternatively, you could just wait until new growth shows, and cut off anything that's not sprouting. It will probably be next spring before you get blooms again.