Just keep a resignation letter with you at all times and fill out the date when you've finally had enough...but NEVER sell out your self respect or dignity to these people.
I agree.
The days of warning letters being used as an instrument to create fear and retain control by management are long gone. The ultimate response to any warning letter is showing two fingers to the company by immediately submitting your resignation on receipt of any childish letter.
EK is merely a job at the end of the day and nothing more.
We are professionals and have worked hard to be where we are in our careers. Never lower yourself to the level where you are treated like a kid who has repeatedly misbehaved in the school playground, where a warning letter is more appropriate.