HP,
A few thoughts for you to think about:
Isn't writing a letter of disassociation playing by their rules and doing what they would prefer you to do? Doesn't it save them the trouble of having to DF you? Isn't it playing into just what they want you to do?
While you may be upset now at finding out about their corruption, lies, etc., what might be the possible benefits of allowing a cooling down period of six months to a year? Will you feel differently then from what you feel now? Will your thinking be clearer and less emotional then?
Some people here have expressed regret at having written such a DA letter, sometimes because it hindered their trying to help others see TTATT. That act closed doors that might have remained open had they acted with more insight into the future.
Just some ideas to think about...