BAPI FOR changing LFA1-SPERM AND LFM1-SPERM FIELD

hello my requirement is as follows

depending on vendor characterictics i need to change the field LFA1-SPERM AND LFM1-SPERM TO 'X'.i have created the report till comparison of data and i have the internal table ready for changing the data but i don;t know which BAPI to use for changing the data in the table.we can see the fields to be changed in transaction XK03->EXTRAS ->BLOCKED DATA->PURCHASING BLOCK.

pls help me out in this .my email id is pritiadukia@gmail.com