You can delete rules by right-clicking on them and choosing the Delete Rule option from the displayed menu.
If Delete rule confirmation option is set (see Confirmations), you will be asked to confirm your choice before the deletion becomes effective. If there are existing instances of the type based on this rule, an additional warning is displayed in the confirmation window.
If the type is present at several different locations in Rules, you will have to delete each rule if you wish to remove the type completely.