So I have recently left a campaign and one of the players used my account to make his character that way the dm could view all of the characters. Since I have left the player continues to use my account and I was wondering is there a way to log them out without being at the session to where they can not access my account?

You can delete his mobile device (if you see it) here: https://www.dndbeyond.com/account/mobile-devices

Changing passwords may prompt a relog on any open sessions (not sure).

Worst case, contact support (link bottom of page).