I don't think that clumpers are immensely powerful and hard to contain. Admittedly, B. fungosa does seem to spread a little further than most clumpers, but it will stop when it runs up against the retaining wall...unless it's a very poorly built retaining wall. I have a large B. fungosa and it's been planted right next to my house for years. It hasn't done a thing to the foundation and I don't expect it to.

However, if you don't want the clump to get larger in diameter, there is no option but to take out some of the growth.

Thanks for the reply, which is somewhat reassuring with respect to the retaining wall. But I did underestimate the size of the clump which is more like 6'x6'. Which means I really need to control its spread regardless of my worries about the retaining wall.

So do I lop off the culms to the ground until I get the size I want (say 4'x4'), and then dig down just enough to destroy the buds? Or do I need to dig out the roots wherever I've amputated the tops?

If you just take out the top growth, it will keep expanding underground. Unless you have a specialized tool like a bamboo spade, the best approach is to cut back all the top growth you don't want so it is out of the way, and then chop down into the ground to sever and remove the roots. This will be somewhat hard and you may need an axe, a pickaxe, and a root pruning saw. A SawzAll with a long blade may help (you will probably go through several blades).