How do you break-up with a close friend for good?

For the past several months, I've been having an issue with a friend (a former girlfriend) who once played a major role in my life, now continues to be a toxic presence. This ranges from ignoring boundaries to being only available when it's convenient for the person.

For those that have experienced such a person--what did you do to restore normality to your life and cut a person like this out? Thanks in advance for constructive answers!