How to stop auto committing of the sales order and commit only the qty from PO which is linked to that SO

You can stop auto-commit of sales orders by changing the default commit option in Setup > Accounting Preferences > Order Management (subtab). The setting is right near the top. Right below that you can choose the setting for the order of commitment by NetSuite's engine (item commitment transaction ordering). You can read more about committing inventory in Suite Answer 32363. You cannot control commitment like you seek, but perhaps you can implement an acceptable solution using scheduled commits combined with saved search criteria to achieve a greater level of control.

I do what Automatech suggest but only on the individual sales order often when we have project orders. This way I do not change that setting for all orders but just for that order. It allows me to change to Do Not Commit or Complete QTY so that I dont have to reallocate everytime someone elses orders. Im not sure if your sales orders are set up like ours but it is an option on the individual sales orders for us.