How to clear a value from a Salesforce picklist?

I've designed an opt in/opt out form that updates data in a respondent's Salesforce profile and I've hit a snag with a picklist field. When a respondent opts out, a picklist value/selection is sent to reflect the reason why. When they opt back IN, I need to be able to clear the picklist value so it's reset back to None/NULL. I've tried several different ways to do this, but they either cause my API call to fail, or they do nothing at all. Does anyone know of a way for me to clear a picklist value/reset a picklist from Qualtrics?
FYI, we're using the out-of-box Qualtrics/Salesforce integration and I'm not able to edit the Salesforce picklist field on the SF side to change the default value or add new options. Thank you in advance!