Follow these steps to close a Service PO (it is the same step for other Po) :

Go to change mode by using T-Code : ME22N

Go to ITEM tab: 

Choose Invoice tab ---> here you will find 4 options. 
- Put tick mark in Final Invoce column.
-- Then save it.
--- This represents that you have closed the PO.

Thisis the Screen shot for your reference:

SAP MM

Read also
Case of Import Purchase Order

SAP MM Reference Books:
SAP Material Management Interview Questions, Certification and Configuration Books

Goto:
SAP MM Purchasing

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SAP MM (Materials Management) Configuration Hints and Tips

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SAP ABAP/4 Programming, Basis Administration, Configuration Hints and Tips