The message being sent is the default.mailtpl, probably two in
conjunction:  SIGNUP1  and &SIGNUP

You can either modify those and put them in your listname.mailtpl
or turn them off in your listname.mailtpl, like this

PUT listname.MAILTPL  PW=xxxxxxxx
>>> SIGNUP1
.QQ

That should prevent the default SIGNUP1 from being sent.  Or, you can
edit the $SIGNUP template to your taste and put that in your
listname.mailtpl

This is covered in chapter 9 of the manual, if you have a copy.

Douglas