The roots are usually close to the surface so dig/chop a circle around the clump to sever all the lateral spreading roots then cut through remaining root ball cut into smaller sections and lever up and out. You may need a chainsaw for the rootball cutting !!!
I agree with ingle, the roots are only a couple of inches below the surface, Dig 'em up. You'll find that they are all joined together so withour clump, we find you can often haul on one and several strands come up from the soil. It's hard work, but it works.
If its a vigorous clump forming species, then digging it out with a strong fork or mattock is preferable.
Weedkiller will most likely run straight off due to the waxy surface of bamboo.