I would do it from a simple segment (one filter with one filter criteria) and a program canvas.
Segment (Create Segment):
* Filter Criteria: Compare Contact Fields
** BR_CLIENT_ACTIVITY not exactly Inativo
Example using different field:
Save Segment according to your naming conventions.
Program (Create blank Contact Program):
* Segment Members - Choose Segment
* Contact Washing Machine or Data Update Rules - I am just going to give instructions for Contact Washing Machine since it can all be done in the canvas
** Configure (double click and click pencil icon)
*** Source = field; Add Action = Set to Blank; Destination = same field
Save and Activate
Once complete, you can leave it running so it will check every 24 hours for those that fall into that segment and clean it up. Otherwise, you can disable it and run it as needed.
OR, if the field doesn't have that many values, in that Fields overview where your screenshot is, you can click the dropdown and Edit each field one-by-one, setting the original value to a new value of blank.
Not an API solution, but a great solution using the GUI, thanks for taking the time to share event with images.
If you wanted to use the API, there are a few ways - you could do something similar to what I did above. But I would probably do a search on the contact field and update the contacts returned
If you search on contacts, you define the field based on the DB name (likely the string+1)
Then you can bulk import the data
Or you can possibly loop through