PO Release Restriction

Dear Friends,

we have release strategy in PO.

if PO Value grater than 50K and lessthan 1.00lakh there preson 1 and person 2 can release PO

if PO Value grater than 1.00lakh there preson 1 , person 2 and person 3 can release PO

i need if Person 1 Release the PO ... Users can not change/edit that PO un till un-release the PO by person 1 or person 2...

how is it possible please advice

with regards,

Rajesh G