How to delete any element once data has been entered?

Once you've started feeding data to any element, it cannot be deleted from within the respective element. However, it can be removed from the activity detail page. (Activity detail page: the page where all the information you've entered has been visualised). In the particular activity whose element you want to delete, hover your mouse over the relevant element block, find the cross button, and delete the entire block. Be informed that you cannot publish your data without all the core elements' data being present in your activity.