To delete an activity, open it and select the delete option from your platform's menu. You'll be asked to confirm before it's removed.
On the Strava Website
Open the activity you wish to delete by selecting its title from your feed or profile page. To the left of the map, select the ellipsis icon and choose Delete from the drop-down menu.
On the Strava Mobile App
Select the ellipsis icon in the upper right corner and choose Delete. If you're using an Android phone with a built-in menu button, you won't see the three dots, use your phone's menu button instead.
You'll be prompted to confirm that you'd like to delete the activity. To learn how to restore a deleted activity, read this article.