Once you've saved your event, the season you've just entered will be associated with your entire team. This means that anywhere you see a season select drop down will be automatically populated based on the seasons you've entered for your event.
If you ever want to delete an entire season, you'll need to find which events have those seasons and either;
a) delete all the events, or
b) edit each event so that it no longer references that season (you can either enter a new season, or leave it blank).
To do this, head to your Team screen and filter the schedule by the season you want to find. You can then choose to delete or edit the associated events.