Disabling Push will most likely stop the excessive data usage. See How to configure Push?
When force stopping an app Android will make sure it won’t do anything until the user opens it again. So it’s not surprising that this will stop the data usage.
Restricting background data usage in Android probably has no effect when Push is active. K-9 Mail is using a foreground service and thus counts as foreground app.
Disabling background sync in K-9 Mail has the same effect as disabling automatic polling and Push for all accounts. It is effective in preventing background data usage, it also means K-9 Mail doesn’t automatically check for new messages.