Deleting select field data using bulk upload or import from CSV

Try putting the word "null" in your import file. 
So whenever the field is blank it is "null".

I haven't done it, but I believe QB will then clear that fields values.

for the excel file you might have to do a conditional column that either takes the true value, but if blank inserts "null".

(I have a hunch it might not work, but its worth a shot)