To delete an activity from the platform, go to Control > Content > Activities and open the activity you want to delete. Then click the three-dot menu in the top right corner and choose Delete.
You need the delete permission to do this. If you do not have it, the Delete option is not shown.
Tip: to find activities that are not linked to any automation, use the No active automations filter in the activities list.
If the activity cannot be deleted, it is still in an Automation.
Click on the activity and go to the Automations tab and check whether the activity is in an Automation.
Open the Automation by clicking on it and delete the activity from the “Unlock activities” action.
Then delete the activity.