To delete an activity from the platform navigate to the Control environment – Content – Activities. Then hover on the activity you want to delete and click on "Edit".

Try to delete the activity using the three-dot button on the right upper corner.

If that’s not working, the activity 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.