What @mikeone has suggested is correct, but it only covers "Push" (And that is disabled by default). Scheduled sync is enabled by default, - and that's what you want to disable to avoid automated email retrieval. It is in the same area under as @mikeone described, just under "Scheduled sync" section.
You may also double check that nothing is enabled in the "Events" section (default is "disabled").
Once you disable the scheduled sync, Push, and on-events sync, the app will not sync that account.
And to manually check a particular account, you can either touch the wheel next to the account in the account-list view, or if you are in the message-list view, - pull the list down from the top and release, - it will sync the folder you are viewing.
If