Hi David,
I assume that by “open a segment” you mean “start from a segment”, i.e. run this campaign for all contacts in a certain segment. Right?
That said: As far as i know there is indeed no proper way to actually delete the value of a contact’s field. The best workaround we came up with is just change it to an “_” (underscore) and test against that value, too, whenever we want to check for “empty”.
I think this is worth a feature request. Here it is: Form Action to PURGE a contact's field value