I cleared out some overgrown Canna the other day
(anyone local want any?... they're laying in the compost pile, lol)
You can grab them by the base (as low as you can) and yank, the rhizome will come right up, and split itself away from the surrounding stems. Some species or cultivars have bigger rhizomes than others, so your mileage may vary
You might need a hacksaw to make it easier, but you'll know right away what your up against.
Whats unfortunate is that the bugs love the canna here and can quickly turn them into very sick looking plants when they eat all the edges of the leaves. It certainly ruins the landscape effect and takes the focus away from the healthy plants...
Different kinds hold up better than others (they must taste different to the bugs).