To Delete a Template

In the Element Templates dialog ( Drawing > Home > Attributes > Element Template > Manage ), expand the open DGN file and the template group.
Select the template that you want to delete.
Do one of the following:

Click the Delete icon.

or
Right-click the template and select Delete from the pop-up menu.

or
Press the <Delete> key.

Note: Templates that have been associated with elements cannot be deleted.