When I worked in front end, simply agree with them and try the best you can to accommodate them, if pandering to them heavily still doesn't work, ask if they'd like to page for a manager. Let the manager take over.

Now that I work in Produce, we get abusive customers too, but it's mainly because they know they can say a slight insult then just push their cart quickly out of the produce section instead of having to stand there like they would during a front end transaction. Most customers' complaints are that something is out of stock or the in stock is not high quality, which we simply get them the freshest from the backroom and replace, or tell them the item got warehouse scratched and we can't do anything about it. I've had some really rude customers though, best thing to do is ignore them while still acknowledging them if you can.