The Fears are virtually unstoppable. The Lonely Hearts have never completely eradicated or killed a Fear. We probably never will be able to kill one. Even if one were to kill a Fear, it's commonly theorized that either the Fear will pull itself back together and come back in a new form, or that Fear's absence in the ecosystem of the planet will cause widespread problems for humanity in the long run.
If I had to compare the Fears to something, I would compare them to honeybees. Though they are quite annoying and often hurtful to humans, they are very essential to the planet, and without them we'd be in a lot of trouble. Okay, that's a lame comparison. But you see what I mean?
Then again, I don't really believe that. I think the only reason we haven't gone through with killing a Fear is because we're scared of the consequences of that. If I had the choice, I always would take the choice of killing them. It's protocol to say otherwise, but to tell the truth I'm working my hardest not to contain the Fears, but to eradicate them.
If we wanted to, and had the chance to, I believe we could kill a Fear. In fact, we've developed technology recently dedicated to that cause. It's known as "Silver Bullet" weaponry, and it's shown to be able to injure Fears, and also kill pieces of the hivemind-type Fears. We're not helpless against Fears as we used to be.
Silver Bullet weaponry was developed using my inheritance as a Rivers child. My grandfather passed down three pocketwatches, that he claimed were blessed and had multiple abilities, including the ability to hurt Fears. It is unknown where he found these objects, or how he created them.
One of my earliest memories is being attacked by a Fear and being saved by my own personal pocketwatch. I've used the watch on multiple occasions. It works.
I believe those watches are the keys to humanity's salvation.