You can remove ESPN Unlimited from your subscription anytime during the one-month free trial to avoid being charged, but please note that you will immediately lose access to ESPN.
Simply log into your ESPN account on espn.com or the ESPN app, go to your Profile > Account > Subscriptions, select "Manage Subscription," and follow the prompts to remove ESPN.
Removing ESPN Unlimited from your subscription will not automatically cancel your MLB.TV subscription, unless you choose to cancel both subscriptions at the same time in your Account Settings.