How we change what others think, feel, believe and do
Make a subject that is particularly embarrassing undiscussable. If the other person brings it up, refuse to talk about it.
This can be applied to individual negotiables also.
Distract them by moving quickly on to a separate and different subject -- preferably one that they will find interesting.
If they persist, a way to prevent them continuing is to give some detail that embarrasses them into giving up.
Sorry, I don't talk about my private life. Would you like to hear about what happened at the party last week?
No, it has to be green...I just want green, ok!!
I can't be at the meeting tomorrow because it's my grandmother's funeral. Ok? Happy now??
Making something undiscussable puts it off the agenda. A flat refusal is often enough, especially if it is repeated as a broken record.
This is a typical method that is used for things that are particularly embarrassing. In groups, it is not uncommon for people to have unspoken agreements that 'I will not talk about your failings if you do not talk about mine'. When a new member enters the group, they quickly learn what not to talk about.
The 'Emperor's New Clothes' is a parable that shows how even obvious things become undiscussable.