You cannot delete markers if vehicles use them as base markers.
To delete one or more markers from your scenario:
Open the list menu in the upper left corner of the List panel and select the Markers option. The Markers list opens.
Select the check box beside each marker you want to remove in the grid, and then click the Delete button on the List panel tool bar. Alternatively, click the More menu button in the upper right corner of the List panel and select the Delete Markers option. The markers are deleted immediately (no confirmation required).