I'm going to break my friend's string and not pay for it
Why would you break your friend's string?
He always mocks my opinions
You shouldn't damage things that belong to friends. Maybe you can tell him that you don't like being mocked? That would be more likely to make him respect your opinions instead of destroying his property.
I don't care. He doesn't care about how he mocks me -- why should I care about his stupid string?
I understand you're frustrated, but what will you accomplish by destroying his property? He certainly won't stop mocking you; he will likely only do it more. Sitting down and respectfully explaining to him how his behavior affects you should evoke more positive response; if he ignores your concerns, it's time to reconsider his "friend" status.