If the parents refuse to sign the IEP then the child or student may not be allowed to attend the IEP meeting or have any services that the IEP provides.
The student in some states needs the parents signature on the IEP form before the child or student can attend the IEP and IEP meetings.
There are 3 states that require a parents signature on the IEP for the student to get IEP services from the school and if the parent refuses to sign the IEP form then the student won't be able to get the IEP services.
There's no law that says that a parent has to sign the IEP so it's perfectly fine to refuse to sign the IEP if the parents are not in agreement with the IEP.
You can't be forced to sign the IEP although the student won't get the services.