"Excessive returns" could be buying and returning much more than you keep or maybe even the degree of inconvenience.
The only person I know who has been banned from Amazon returned a tv three times but there may have been a previous history issue as well.
I imagine an algorithm has been developed to calculate how profitable an online consumer is.
It doesn't bother me, I try to be as discerning as possible when making purchases and consider reduced returns beneficial to all.
There are so many places to shop I would be unfazed should I be shut down.