The question is what do want turn off?
If you haven't synchronized then go to Avast->Settings->Passwords cancel login dialog and you will see Forgot master password option on top. Using the link you can remove your current password storage, all passwords int there will be lost!
You may also want to remove browser extension from your browser if you no longer plan to use our password manager.
If you have synchronized then those steps are more complicated, I will post you instructions if you confirm you can't use steps mentioned above
