Is there a way to stop being offensive to a certain group of people?

In the presence of people I see as obese, I catch myself saying things which I prefer to think is completely unlike me: covertly vicious. The first time I can recall, but there may well have been earlier times, was about 10 years ago, when I made two comments in a row, which turned a newly met friend of my good friend into a long-term enemy, and for a reason; I would not have forgiven such a behaviour to anyone either.

Whereas this does not happen very often, I do end up saying things I instantly regret. This happens even if I find the person very amusing and likeable.

How can I stop myself doing that? Not only is it legally classified as harassment, but also ruinous to my relations with some people and their friends. If I were religious, I would be ready to swear to God: I do not want it; I take responsibility, but these are reflexes.