When you setup an email account in Mac Mail, it will automatically be configured to download new email messages based on that email program setting (how to change how often Mac Mail checks for new emails). But, short of actually deleting an email account you no longer use (temporarily or permanently), you can customize that email account to be excluded from the list Apple Mail automatically checks, but also excluded from your manual checks for new messages. Since Mac Mail can support multiple accounts, this allows you to concentrate on your main account, and only activate another account as needed (Mail doesn't let you customize send/receive settings on an account-by-account basis, so all of them will be checked for new messages at the same time, but the last section of this tutorial explains how to exclude an account from automatic checks for new messages - without disabling it!)
When you have customized your advanced email account settings, hit Esc on your keyboard to close the Preferences dialog, and hit Return (or click "Save") to apply your new options and return to the Apple Mail main window.
Tip: as mentioned throughout this tutorial, enabling or disabling an email account, or including / excluding it from the automatic checks for new messages, is only a temporary measure. You can re-enable or re-include the disabled or excluded email accounts in question at any point, by following the steps explained above!