According to RFC 2821 section 4.5.1, EHLO MUST be supported. You cannot
disable that command and remain conformant.
Of course RFC 2821 doesn't specify how to violate RFC 2821.
In the general case, you can disable commands by not announcing them in
the EHLO response. If the implementation supports ETRN but the admin
disables it, you simply leave ETRN out of the EHLO response.
Arnt