A very short answer to this: you can't. And you shouldn't need to prevent this from happening.
A signature is simply a mark. It could be as simple as an X. It actually proves nothing and merely indicates that the "signer" processed the document. It can be argued that the handwritten signature is a memory aid for the signer to remember that they had signed it.
This is why legal documents include a witness to the signature. A 3rd party that can attest that the signer was the one who signed.
And this is the step that a digital workflow needs to include.
The signature/mark is never the point, but rather the 3rd party witness is. The system authenticates the signer and timestamps when the signer signs. In this way, it shouldn't matter if someone can copy the signature or whatever pretty picture someone draws on the digital pad (I know an infosec pro who signs everything as "Donald Duck").
"Digital signatures" (the cryptographic ones) offer more in terms of verification without user authentication at the time of signing and more portability outside of a company's systems, but that requires more of an infrastructure to handle that and to verify signatures. From what you've described, you seem to simply need an authentication step from your office systems when someone draws a pretty picture.