Honestly, it sounds like the individuals have access to your computer and/or the email account tied to your Delta profile.

If they are repeatedly breaking into your account after you change the password then it suggests they have access to an external source to get back into your account, or they have access to your computer and read your password when you change it.

I would try using a different computer than what you have been using. 1) Change your password on your email account and enable 2-factor authentication if possible. If you are not already using Gmail, consider creating a new Gmail account and take advantage of their 2-factor authentication security feature. 2) Look into whether your computer is compromised, it may need to be wiped.