If you don't want the default signature to be used in any new email messages, just delete either the actual signature or change the setting so that Outlook does not use any of signatures regardless of how many are saved in the profile.

You do this through the Tools > Options menu then select Mail Format and click Signatures. Either select the signature and click Delete OR change the New messages drop down menu to None. Click OK till you are back at the Outlook screen again and try creating a new email message. You should get a blank message with no signature.