Not very easily, I'm afraid - these plants sucker anyway, but if you take down the main stem, it increases the suckering activity, as you've discovered. Your only option is to dig around and expose the woody roots wherever they are and drill into them to make small holes and apply SBK to the holes. SBK is a brushwood killer, but don't get it anywhere else, especially not on the soil. Cover the holes after application. You can cut down any whips or long stems coming off the roots.

Oh dear! Thank you, sounds like a job and a half its all over the place.

I know how bad they get - my sister had the same trouble...