Provide capability to import blank values for dates and list formatted line items

I'm not sure if it works for your needs, but here is how I would accomplish this partial truncation:

1. Setup a second data module which uses the same dimensions as your main data module, but contains only the line items that need to be cleared. Hardcode the line items to BLANK and save a view of that module.
2. In your main data module, import the BLANK data view to truncate the specific line items that need to be removed.
3. Bundle the BLANK import action and the new data import action into a Process, so the BLANK truncation step runs before the new data import. Setup your data import so it does NOT clear existing data before loading, and it should preserve your existing records while sending new data to line items that were blanked out.

Another option would be to setup two data modules, one that is held more constant (the data you want to save) and one that is updated frequently (the data you want to blank out on the import). With this case, you can keep both modules updated independently, and then join the results together in a staging module or report module.