With Set Actions, the user has three options when "clearing the selection". These are "Keep set values", "Add all values to set", or "Remove all values from set". As an example, I click on Ohio in a US Map and it adds Ohio to the set. When I unselect Ohio, then I have the option to add all states to the set, remove all states from the set, or simply keep it set at Ohio.
Conversely, with Parameter Actions, once you click to set the parameter, there is no options for clearing. That value remains until you click on something else. Let's use the same example as before. I click on Ohio in a US Map and it adds Ohio to the parameters. I unselect Ohio, but Ohio will remain in that parameter until I click another state.
I would suggest that an option be provided that is similar to Set Actions. When building the parameter actions, you would have options on what to do when the value is unselected (or cleared). The options would be "keep parameter value" or "set parameter to another value". So if it were set to "keep parameter value" then it would work as it currently does; click Ohio, it sets the parameter to Ohio, then unselect Ohio and Ohio remains in that parameter. However, if you set it to "set parameter to another value", you would be able to feed another value into that parameter (such as NONE). So, you select Ohio, it feeds Ohio into the parameter, then you unselect Ohio, and it feeds "None" (or some other value) into the parameter.
(As a side note, to do this now, you must set up a "fake" value or button with a new parameter action to set the parameter to another value).