Right-click in a file explorer, context menu > Properties, make it read-only...
Or the good old chmod -w on Unix systems.

Guess I should clarify, I want to write data from a serial port to a CSV file created in processing.  I want to make sure the data within the file cannot be modified or deleted.

If you make it read-only, you cannot write data in it...
If you want to lock it after the sketch has run, my advice still stands.