There's no way to clear events you've sent. However, if you've only been working in Sandbox so far, don't worry! Sift doesn't learn from Sandbox accounts, so these unwanted events won't affect your score quality.
If the number of unwanted or incorrect events you've sent to Production is fairly small, there's no need to reset. The good events will quickly overwhelm the unwanted ones. If the number is large, you may need to set up a new Sift account with a different email address. If you're not sure, contact support.