To Delete a Template Group

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

Click the Delete icon.

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

or
Press the <Delete> key.

Note: Template groups that contain templates that have been associated with elements cannot be deleted.