How to secure part of a document after last signature

I have a document that has 5 signatures that, depending upon the dollar level of a certain field, may or may not be required.  How can i set the form to lock part of the form upon the first signature but to keep the other 4 signature fields available for other signatures, in addition to another section of the form?

I am using java scripts to determine which signature is required, by turning the box around the name green or red depending on a certain dollar level.