You can't prevent users from creating more than one digital ID, there are many tools\utilities available that can be used to create digital ID's (including Acrobat)
You could use the "seed value" feature when creating the signature field with Adobe Designer. You can restrict which certificates (digital id's) can be used to sign the PDF (see attached SigningCertificates.gif). The problem in you case (when using self-signed certificates) would be that you would have to identify each and every one of your user's certificates in the signature field configuration.
You might want to look into setting up your own certificate server (there are open source servers available), this way you can issue the digital ids (certificates) to your users. This also means that you can specify in the signature field configuration that only certificates issued by a specific certificate server can be used to sign the document (see attached IssuersAndPolicies.gif).
Hope this helps.
Steve