Users can edit the deployed email signature when composing an email in Outlook.
Users cannot save changes to the deployed email signature in the Outlook Options dialog.
If a user edits the email signature when composing an email, then the email signature will revert to the deployed signature on the next email the user composes.
The only way to avoid the possibility of users editing the email signature when composing an email is to inject all email signatures on the server. However, then email signatures are not visible when composing email, and this may lead to users writing their signature when composing email.