With perseverance!
Cut it hard back to the main stem now (or as soon as you want) then in spring spray the new growth with a strong systemic weedkiller - use a 'brushwood' strength. Depending on the age of the plant it will probably take several applications to get rid of. Keep it in check throughout as well so that you can be sure that you are spraying the whole plant.
Don't try to dig it out as any bits of root left will sprout into new plants.

I also take a large drill and drill a 2 inch deep hole in the centre of biggest trunks. top up hole with rootkiller or brush wood killer cover with bag and brick stones. usually I only have to top up a couple of times to effect a death. however I do usually do this at the start of the growing season.