No need to disable iCloud sync.
The things iCloud will sync are:
- Safari passwords and bookmarks
- Keychain (passwords and certificates)
- Desktop & Documents
- Various applications if you permit them to
So, for passwords, don't allow Safari to save your password for banking sites. But any other browser passwords or bookmarks can sync normally.
For documents you want to keep exclusively on the MBP, put them in a designated folder other than Desktop or Documents, for instance ~/banking where ~/ is the shortcut to /Users/yourusername/, in other words your home folder. As long as your banking documents go elsewhere, they won't sync.