What do I do when the parent will not sign the IEP in disagreement or agreement?

A parent is a mandatory participant of an IEP meeting. While parent input is required and important, it is not required that a parent sign an IEP in order for it to be implemented. 34. C.F.R. 300.321: 71 Fed. Reg. 46,682 (2006). 

There is one exception: if a parent does not agree that their child should be assessed or found eligible for special education services, they can withdraw the child from the entire special education process.