I don't think it's possible. I couldn't empty it with the on-board methods and services.
I've also tried using an Expression Widget to overwrite the state and set an Output forces a non-selected state for the Radio Button Widget - but haven't really figured it out yet.
I'd say it's a limitation to the Widget, that once it's set it can't be unset.
Depending on the use case you could add an additional state (selected by default) e.g. "No Selection" - and if this is selected, it will not change / trigger dependent actions...
Seems like there already exists an enhancement for introducing this functionality.
