It is not possible to clear your data and settings created or configured during your kintone free trial. If you wish to use an environment in its initial state, where no data has been created yet nor have any setting changes been made, sign up for a new free trial.

You can sign up for another free trial even if your current free trial has not yet ended. Even after your current free trial ends, you can sign up for a new free trial. Moreover, you can sign up for a free trial as many times as you like.
Note that it is not possible to set the same subdomain for your old trial environment and new trial environment.
For details, refer to Is it possible to restart or extend the period of the free trial?