You can't delete the rows showing your activities and projects in the Time Card screen; however, you can clear the rows or remove blank rows from the grid. Time Card allows you to auto-fill the most recent projects and activities based on the setting in User Settings > Time Settings. So, if you do not want any or want less number of projects-activities pre-filling your time card, you can change the number under the 'Auto-fill' option accordingly or simply turn it off.