You can’t hold them to the terms of the Performance Improvement Plan if they deliberately refuse to sign it. But what you can do is:

1. Negotiate with the employee to secure his/her consent and signature–find out why he/she’s refusing and try to address the issue;
2. If negotiation fails, discipline and perhaps even terminate the employee for refusing to sign. This is especially true if the PIP is part of a fair and reasonable last-chance agreement giving the employee an opportunity to save his/her job.

Hope that helps. Glenn