When there are multiple line items in a single document need to remove payment blocks for all line items at a time .below is the process how to remove block using Mass Change option in FBL1N.
Note:payment block will come from payment terms below are the setting for default payment block

If the document is related to Vendors use -FBL1N, Customers use-FBL5N and GL use –FBL3N.
Here I am showing Vendor related document.
Execute FBL1N:
Go to Dynamic selection screen select document number

Click on Execute. you will see the list of document line items posted to different parties.
Got to ‘EDIT’ select all option.

System will select all line items

Now Click on mass change button
then You will see a pop-up window

To remove the block click F4 for payment block field and select free for selection

And then click on execute
changes button .
After changing you will get a message
.
Regards
Sada Bandla