Hi Santhosh,
To make column as readonly, you need to set the read only property of the cell editor of column.
As suggested by Kiran, get the cell editor of column by using method get_table_cell_editor ( ) and set the read only property by using set_read_only ( abap_true ).
Note:
The cell editor of a column may not be always a input field, find out the type of cell editor like input field, drop down by key, drop down by index, text_edit, etc. Accordingly get the editor reference into respective object.
Hope this helps you.
Regards,
Rama