Hello Robert,
Even though Nishant's solution is more elegant, the easiest way to achieve this is to add an extra hidden (it's not visible in the UI) attribute to your context structure (preferably type WDY_BOOLEAN) and bind each table cell's read-only property to that attribute. Afterwards, depending on which table line you want to make read-only, you set the property of that attribute to abap_true.
Hope this was helpful!
Tudor