Assuming that the ads are in line with policy and guidelines - having advertiser's ads running is to the benefit of everyone; users, advertiser and Google alike.
With that principle in mind, it naturally follows that Google would not close down advertiser accounts without an good reason to do so. Why turn down the revenue without a good reason?
The good reason, in a broad sense, amounts to protecting both user experience and the integrity of the AdWords system.
In my experience the most common reason for an advertiser to be 'banned' (as the original poster puts it) is a consistently demonstrated lack of regard or concern for AdWords policies and guidelines.
netmeg and wrgvt are right on the money.
AWA