Sensei's Divining Top activations
No they can not look at the cards again.


  1 : Look at the top three cards of your library, then put them back in
  any order.


By placing the cards back on the library they have finished taking all the actions the  ability allowed and then it finishes resolving.

If you were playing a very competitive game, this is not a legal thing to do.

I do not think there is any way to interpret the above rule and all its parts to allow a player to re-do the last step of an ability.

Furthermore the rules do not have anything in them about a player stating that anything resolves. A spell resolves when all of the steps listed in its text have been completed. 


  608.2c The controller of the spell or ability follows its instructions in the order written...