1. Reduce the communication.
As much as possible, you should not be talking to them. Limit your calls and e-mails. It may be hard at first, trying to ignore the person, but you have to remember unless you do so they will not stop bugging you.
2. Talk to them.
You may want to confront them over their bad behavior, particularly on what you feel about and see on them. Give them the benefit of the doubt. Usually a lot of them are not really conscious of what they're doing. It takes persons like you to make them realize their mistakes. Determine the best time you can talk to them. Make sure that you limit the issue to his behavior. Do not attack the person.
3. Be real.
If it's definitely your decision to end the friendship, you can tell them first hand. They may tell you that you're unfair or unselfish or even call you names, but it's actually a lot better than leave them hanging in the process. You don't want them to be second-guessing. This will also hopefully serve as their wake-up call and evaluate their negative personalities.
4. Detach yourself when it comes to emotions.
You can listen to them; you can even empathize with them. However, you need to get those walls put up around you, especially if they are already damaging your credibility and self-confidence. Learn to ignore those claims they cannot substantiate or those that you know are definitely wrong.
Now if your confidence gets shaken up you can just use the subliminal messages to help you get it back in full form. Here are some of them:
I won't allow negative comments to hit me.
I won't allow toxic friends to determine who I am.
I know who I am.
I know myself a lot better.
You can repeat these subliminal messages as often as you can. They become more powerful when you are so down, and you don't have anyone to pull you up immediately. These will also strengthen you, so you don't get affected of what other people are saying about you.
5. Surround yourself with better friends.
Meet new ones, and choose them well. Select those who have positive outlook in life and who share the same interests as you. These should also be individuals who you think can surely make you a much better person. Hang out more with them than with your toxic friends.