
* E-mail notification is very important in this kind of things. People need to know what's happening without checking the ladder page every day. Every day will soon turn into once a week and later on, once a month and finally when they remember it exists... So, good you're working on this.
If possible, provide e-mails with full explanation in them of what the player is supposed to do and how to do it, depending on the situation you're sending the mail for..
* Banning accounts is rather brute, don't you think so? I would suggest an account becomes "inactive" after two months of not logging in. Inactive accounts are not shown on the mainpage (maybe provide a link on it to go to inactive accounts). When accounts are set inactive, none can challenge those players and all the pending matches get declined.
If ones account is set inactive, notify him by e-mail but give him a chance to activate his account again.