Prevent email signature from being inserted when replying
I believe you can create a template for a new email (which would include you signature) and then leave signatures off everywhere else by setting our signature to none.
The template can be a draft message or a sent message. Either way you have the option of sending them again, which makes them de facto templates.
With MacMail the only way to do this is to set the default to no signature, and then use a program like TextExpander to insert a signature for you.
You CAN do this in Mozilla Thunderbird however.
Good luck B