Having just checked out my bank details online, I thought I'd examine my credit limits. There is a box on the screen which says "Ammend credit limits". Since I have a ludicrously high credit limit, I thought I'd lower it by several thousand. I was therefore slightly astonished that although I could move my credit limit "up" online, I had to phone a telephone number to "lower" it.
No wonder consumer debt is such a problem when banks make it so easy to get in to debt but make it so much harder to be careful.