I'm not sure about the signature pad, but for form fields, follow these steps:
Go to Review | Restrict Editing
In the pane on the right you can leave the first box unchecked.
Check the second box and choose "Filling in Forms" from the drop down box.
Click on "Yes, Start Enforcing Protection" and create a password, if you want to discourage someone else from editing the rest of the document.
That should do it for you!