This is a common request I run across. You’ve got a selection list and you need to reset it back to ‘Not Specified’ for whatever reason.
Although I swear I’ve done this before, every time it comes up I’m scratching my head trying to figure out what I did the last time it came up.
As I was looking for help on another issue I ran across Doc ID 2478146.1 on MOS that spells out the ‘correct’ way to do it, leaving it here for my reference:

Some entities involve a Collection, or group of values it can be set to, with an example being the PreferredPhone (within the Candidate entity) relation to the PhoneType entity. However, though there can be only one preferred phone value at any given time the relation would not seem to be a Collection which is for one-to-many or the opposite. However, the Recruiting data dictionary shows the PreferredPhone relation on the Candidate entity supports “Reset”.
This means the value can be set back to ‘Not Specified’ by passing the current PhoneType value that needs to be removed and selecting the options “Remove Element from the Collection” and “Start a new element in the Collection” – this will restore the value back to ‘Not Specified’.
There are other entities with similar relations where the same method can be used to change the value to ‘Not Specified’ the key being that the entity supports “Reset”. Another example being JobInformation entity relation to EmployeeStatus, Code.
Please see the Recruiting data dictionary for the version of Taleo in your zone to determine if an entity support Reset