(global-unset-key (kbd "C-z")) might do the trick ....
That doesn't sound too good.
I don't use CUA mode (never have), but looking around a little
gives me the impression that you can just customize this option
to nil: `cua-enable-cua-keys'.
,----
| cua-enable-cua-keys is a variable defined in `cua-base.el'.
| Its value is t
|
| Documentation:
| Enable using C-z, C-x, C-c, and C-v for undo, cut, copy, and paste.
| If the value is t, these mappings are always enabled. If the value is
| `shift', these keys are only enabled if the last region was marked with
| a shifted movement key. If the value is nil, these keys are never
| enabled.
|
| You can customize this variable.
`----