Can you lock one filter in a pivot table?

[p45cal]

The problem is the source data has to be in the file.

Then your cause is lost.
Anyone with a little nous can do some internet searching and break any protections on the workbook, then they can create their own pivot from the data.
Can't you restrict the data to just the relevant departments? I realise this might mean some measures or calculated items/fields might not show correctly if they use company-wide data.