You will need code to create If(), boolean logic & flag checks. Then using Blynk.setProperty(vPin, “property”, value) you can “black out” a button to visually indicate an inactive state.
Not really… all logged in or shared users to the same project are treated as one, as far as widgets are concerned.
But I guess you could use Terminal to assign user codes that could be compared in code to sorta determine the correct user.