I had a toxic friend for about 5-6 years. I'd cut her out of my life, but always let her back in, in the hopes that she'd changed. She never did.
It took a long time, but I finally realized that I couldn't offer her enough support & encouragement for her to turn around some bad decisions in her life - the more I gave, the more energy she sucked out of me.
I didn't want to be an ass & I didn't like the feeling that I was 'throwing away' someone who was no longer useful to me. I stuck out the friendship for a lonnnnng time trying to avoid that feeling! I finally came to the realization though, that she lived for drama & turmoil - she fed off the attention and pity it got her. I didn't want to live my life that way or be a party to her endless drama-thon, so I finally told her to just pretend I'd been part of her vivid imagination. She wasn't to call me, to come by my home, to stop by my work, or to call my parent's home looking for me - I told her she was dead to me & I needed to be dead to her. It actually felt good to be so vicious & mean to her... and I'm still ashamed to admit that, but it's the truth.
Anyway, I've had to remain firm about my decision because to this day - some 5 years later! - she will still occassionally call my house when she's in town, trying to reconnect with me.
I wish you the best in whatever you decide to do, whether it's let your ties gradually slip away or brutally whack them.
In the end, if someone's not contributing to your life in some way, shape or form - cut 'em loose.