If this is their policy, I would think that they would at least have to warn the customer that the next time they try to return they will not be able to. If their systems flags "no return" at a certain number of purchases,then it should be able to flag at the number right before that so the customer can be made aware and purchase (or not) accordingly. JMO.
Although, I think you (not you personally, but a general "you") would realize if you've been returning a lot of items at a high rate. In that case, I don't think there should be a pre-warning about the next return being a bannable offense. I tend not to return a lot only because I find it a hassle. As much as a love fashion, I don't like shopping nearly as much. So, when I hit up a store, it's usually because I'm need of a particular item--jeans, shoes, sweater, skirt, dress, etc. (With clothes) Sometimes I'll try on, other times, I won't. If I bring the item home, and try it on and don't like it, I may return it. But, that doesn't happen too often.
As for online purchases, again, my patience does me in. I've ordered things online (usually one of, though), and when it comes, if I happen to not like the size, it may end up taking space in my closet only because I don't want to go through the hassle of going to the Post Office and having to wait an extra 7-10 business days (plus the time it takes to mail the returned item) to receive the correct item. If it doesn't fit and I don't want it in my closet, I'll usually ask a friend if they're in the mood for a dress (because it's usually a dress purchased online that does me in (since I have a large bust and a smaller waist, the fit needs to be just right).
Also, the difference for me may be that these are not outrageously expensive items. My wallet can't handle that at the moment.
(As an example, the dress in my avatar is one that I purchased from Urban Outfitters online. It's a bit shorter than what I would normally wear, but I didn't want to return it, so I decided I needed to make it work.)