Is the list box a bound or unbound control? If its unbound then you can set the value to null when running the code on your refresh button.
As for your other button I would run some code in the OnChange event of your listbox, something like...
If me.listboxname.value = "thevalueyourlookingfor" Then
me.nameofcommandbutton.enabled = True
me.nameofcommandbutton.enabled = False