You can delete individual responses, all the responses from a collector, or all the responses for an entire survey. If you delete responses from a shared survey, they're also deleted from any accounts with shared access.
If you have a Basic (free) plan, the number of responses your account can have and delete depends on when your account was created and your billing location. For example, you may be able to delete 40 or 50 responses total from your account.
Deleting responses is permanent. Once you delete them, they can't be restored.